It's entirely possible that I'm either making an ignorant point here or an obvious one. If so, I apologize—I'm definitely in the pro-sabermetric camp, but I'm not an expert on how things like WAR are actually calculated.
So, in brief: in discussions of players, people around these parts tend to heavily emphasize WAR in comparing players—which makes all the sense in the world. WAR is built to compile all the things a player does to help the team win, stripping them of any context that they weren't in control of and crediting them only for what they actually did. It seems to be the best tool available to answer the question "which of these guys is the best baseball player."
However, it also seems to be invariably assumed that WAR is also always the best measure of which player will provide the most benefit to the team at a given position—insert the guy you can get with the highest WAR, full stop. However, it would seem to me that since the outcome of the game is dependent on the outcome of situations in context, the team is sometimes better off with a player with a somewhat lower WAR if their particular set of skills is more complementary of those of the other players on the team.
Specifically, I'm thinking of Will Middlebrooks here. It's no secret that he's not great at drawing walks, and thus has a low OBP and, therefore, a WAR that isn't all that stellar, despite his power. If we considered him next to a defensively identical third baseman who hit lots of singles, the odd double, and drew lots of walks, but had little power, it's fairly likely that the other guy would have a somewhat higher WAR—that he would do more to help the generic team than Middlebrooks would. However, since the Red Sox have lots of high-OBP guys on the team, but few real power threats (really just Ortiz and Napoli at this point, although Xander will hopefully join them), might Middlebrooks actually be more valuable to the Red Sox in particular than that other player would be? To put it more generically, since the Sox generate more baserunners than almost any other team, don't they have a higher expected value attached to each home run, and thus an incentive to attach greater importance to them than a club without comparable on-base threats?