The following is not and indepth study but it is a close examination of it effects on pitchers. I beleive this was done by a writer in the Atlanta Journal.

The World Baseball Classic Effect - Pitchers
Monday February 4th, 2013
Should Atlanta be concerned with Kimbrel pitching in the WBC? (US Presswire)

As the baseball world prepares for the latest iteration of the World Baseball Classic, once again the discussion of the reluctance of marquee players playing for their countries at the international level rears its ugly head. Both players and professional teams alike routinely balk at participation in the Classic, in spite of the gravity of playing for your country on an international stage. The primary reasoning behind the lack of star quality players performing in the WBC is the potential for injury, and the ramifications those potential injuries might have on a player's professional career.
Is there merit to this notion? Are the games and innings of the WBC that detrimental to the regular MLB season of a player?
I decided to take a look for myself, starting with pitchers from the WBC US entries of 2006 and 2009. For each WBC pitching staff, I looked at a handful of pertinent stats for each pitcher, comparing the year prior to the WBC, the year of the WBC, and the year after their WBC appearances to see if there were any significant changes in performance. With this, I also looked at injury histories of those years, again looking for drastic increases in games lost due to injuries, assuming the increased workload arising from WBC participation could lend a player to increased potential for injury.
Here's what I found, looking at all pitchers, for the 2006 and 2009 WBC staffs for the United States (pitching stats courtesy of Fangraphs, injury data courtesy of Baseball Prospectus, and their Player Cards):
Year IP H/9 K/9 BB/9 HR/9 BABIP ERA- ERA FIP xFIP WAR Games Lost
2005 1422.1 7.19 8.72 2.74 0.55 0.273 57 2.39 2.91 3.43 2.7 32
WBC 2006 1249 8.14 8.23 2.75 0.84 0.283 78 3.49 3.57 3.91 1.8 94
2007 1144 8.64 7.95 2.94 0.92 0.291 89 3.96 3.91 4.00 1.3 162
2008 1530.1 7.7 8.28 3.52 0.74 0.282 74 3.15 3.62 3.78 1.5 138
WBC 2009 1312 8.24 8.27 3.77 0.72 0.294 91 3.85 3.63 3.96 1.4 449
2010 1240.1 8.59 8.88 3.43 0.9 0.306 103 4.19 3.67 3.66 1.4 544
I leave it to the reader to peruse the rosters of each of these WBC squads for further information - they can be found here and here. Briefly, the 2006 pitching staff had an average age of 30.4, with a range of 22 to 43. 2009 had an average pitcher age of 29.9, with a range of 24 to 36. The 2006 staff had 14 players, 10 of whom were relief pitchers; 2009 had a roster of 15 pitchers, 11 of those relievers. With respect to WBC innings pitched, the 2006 team threw 48 innings, with the 2009 squad clocking 67.2 innings in the Classic; individually, 2006 saw Roger Clemens leading the staff with 8.2 IP, with Roy Oswaltleading the 2009 staff with 11.1 IP. Overall, both the 2006 and 2009 WBC entries had most of their staff throw a smattering of innings, in the 2-5 IP range on average.
Overall, there is a trend towards decreased production the year of and following WBC participation for American pitchers, with the year after a WBC being the least productive and injury prone for a WBC pitcher. For the 2006 staff, a 12% decrease in innings pitched the year of the WBC, and a 20% drop the year following is seen. The 2009 staff has a similar dip, with 14% and 19% drops in innings pitched in WBC years, and the year after the WBC, respectively. Comparing the two WBC squads, there is a significant difference between the two with respect to games lost to injury, with the 2006 team appearing to be the more healthier across all years of interest. 2009 pitchers were decimated by injury before, during, and after the WBC, with the season ending shoulder injuries and surgeries of J.P. Howelland LaTroy Hawkin dominating the 2010 injury numbers. As a side note, the injury data I collected and used for this article only included games lost due to injury that could feasibly be due to performance-related wear and tear; therefore, games lost due to things like a stomach virus, or a finger contusion from trying to bare hand a come-backer, weren't included.
Speaking of relievers, let's break down these numbers a little further. Here are the same stats as the previous table, broken down by reliever and starter:
Year IP H/9 K/9 BB/9 HR/9 BABIP ERA- ERA FIP xFIP WAR Games Lost
RP 2005 772 7.17 8.95 2.87 0.55 0.275 55 2.37 2.91 3.45 1.8 23
2006 710 8.1 8.29 2.74 0.84 0.280 78 3.51 3.56 3.91 1.3 87
2007 616.1 8.6 8.12 2.89 0.94 0.290 88 3.96 3.87 3.98 0.8 158
SP 2005 650.1 7.24 7.98 2.32 0.56 0.267 61 2.46 2.92 3.33 5.8 9
2006 539 8.27 8.04 2.8 0.81 0.292 80 3.42 3.61 3.92 3.4 7
2007 527.2 8.79 7.42 3.12 0.87 0.295 91 3.96 4.04 4.08 3.0 4
RP 2008 752.2 7.53 8.61 3.84 0.61 0.285 71 3.01 3.45 3.73 1.0 61
2009 652 8.15 8.62 4.27 0.59 0.299 90 3.82 3.52 3.94 1.0 281
2010 518.2 8.92 9.55 3.86 0.81 0.326 109 4.38 3.53 3.54 0.8 447
SP 2008 777.2 8.17 7.37 2.66 1.1 0.273 83 3.53 4.09 3.93 2.9 77
2009 660 8.48 7.31 2.41 1.09 0.281 93 3.93 3.93 4.03 2.6 168
2010 721.2 7.78 7.22 2.35 1.12 0.258 89 3.71 4.00 3.94 2.7 97
While we see the same trends, in general, as the previous table, it's obvious to see that relievers suffer the most when it comes to post-WBC MLB successes, both in terms of performance and injuries. Starters from both squads do a better job of avoiding the post-WBC injury bug, as compared to their relievers, but nonetheless, suffer a drop in innings and production. It is of interest to see how little the 2009 WBC pitching staff suffered with respect to Wins Above Replacement (WAR) over the years before, during, and after a WBC; 2009 starters actually had in some respects a bounce back year in 2010, with their 2010 average stats just as good, and in some instances, better than their pre-WBC numbers. In spite of hints of their usual, staid output, both WBC squads experienced significant reductions in MLB innings pitched and increases in games lost due to injury from the bullpen.
So what does this hold for the 2013 iteration of the WBC? While a small sample size still prevents us from making any rash predictions regarding the 2013 and 2014 MLB seasons for those selected for the WBC, there is enough happenstance to cast a weary eye towards the likes of Kansas City Royals reliever Tim Collins and Atlanta Braves closer Craig Kimbrel, and closely monitor their WBC and spring training performances. While it can be assumed that 2013 will be within their normal levels of productivity, how they are used in the WBC could play a role in their 2014 productivity and success, if WBC history lends itself to be true.
While the honour of playing for and representing your country on the international stage should never be downplayed, the numbers from the first two WBCs do lend merit to the balking of players and professional teams when it comes to participation in the WBC, on the grounds of future injury and performance issues. While the debate of when the ideal time of season the WBC should be played will continue long after the games of 2013 are over, for now, fans will have to be content with the rules in place when it comes to determining international bragging rights for who is the best baseball country.
As for relievers, beware of the World Classic.

The following is a newer study and opinion

Yes, we're still arguing about World Baseball Classic and injuries

By Rob Neyer on Jan 17, 2:50p7
<img alt="" size_name="large_730" src="" />Chris Trotman

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So it looks like, once again, Team USA: World Police will be notable for the absence of some great starting pitchers. All because of a bunch of ninnies are afraid of getting hurt, or something. Right?
Huh. I wonder if @MLB_PR will be re-tweeting this FanGraphs study:
After looking through the statistics of those who appeared in both WBC tournaments, it is my belief that pitchers who participate in the WBC, especially starters, are far more likely to see a regression in their performance, get hurt or both than pitchers who do not play in the WBC. I reason that the most likely cause is the tournamentís timing disrupts the normal routine of pitchers and their arms are not yet ready to handle the stress and intensity then. With data collected from various sources, I will demonstrate the stark differences between WBC pitchers and their counterparts who did not participate in the tournament, using spreadsheet data and graphs included in this analysis.
That's from a piece written by Michael Echan, and titled "Flooring the WBC: How the World Baseball Classic Negatively Affects the Health and Performance of Pitchers".
But first let's go back to Jayson Stark's piece, which does not focus specifically on pitchers. Stark's column, by the way, has a different sort of title: "Busting the WBC injury myth" ...
MLB has done extensive research about the WBC and the impact it's had on the health of players who took part in it. And you know what that research determined?
That the health risks of participating are more myth than reality.
... I'm not saying the WBC is risk-free. I know all about Daisuke Matsuzaka's issues following the '09 WBC. I know all about Edinson Volquez's Tommy John surgery and Jake Peavy's ankle problems.
But here's what I think that data above reminds us: Baseball players will get hurt. Period. No matter what they do. No matter where they play.
Of the 10 highest-paid pitchers in baseball last year, half of them spent time on the DL -- in a non-WBC year. Eight of them have visited the DL at some point in the last two seasons -- neither of them WBC years.
Look, there's a simple truth here that I wish Stark had mentioned: Major League Baseball generally, and Bud Selig specifically, have a HUGE rooting interest in the financial and artistic success of the World Baseball Classic. I think it's fairly safe to suggest that even if MLB has done "extensive" research on the WBC, any results of that research that reflected negatively on the event simply wouldn't be released to the public.
The problem for Major League Baseball is that it's not difficult to conduct this sort of research. I'm sure that teams have done the research, and that some agents have done the research. Anybody who really wants to know or needs to know, can know. Pretty easily. No secrets are being kept from the stakeholders who really care.
Echan's research has led him to conclude that starting pitchers in the WBC are likely to suffer ill effects, in the forms of both debilitating injuries and slight performance downgrades. He doesn't show any regression analyses, so I'm wondering if the results are symptoms of small sample sizes; after all, we're not talking about a great number of pitchers. I wouldn't want my favorite team's ace throwing for Team America or Team Dominica or Team Venezuela, but that's just because I'm cautious and I prioritize my favorite team over all other considerations.
But there are other considerations, and they compete with one another. And there's really no right answer. If pitching in the WBC increases injury risk by 5 percent and decreases performance by 3 percent, is that enough to keep you out? What about 3 percent and 1 percent?
We'll never nail down the percentages. There will never be enough pitchers throwing the same number of WBC innings to get a truly accurate read on the effects. Which is why we'll keep seeing studies that show us what we want to see.

there doesn't seem to be much printed on the positiong player the main focus group is pitchers. I certainly don't think we could argue given the effect on one Dice K!