Dave Cameron makes a very interesting observation over at Fangraphs. To use Dave’s example, when people say stuff like “David DeJesus is a very good center fielder, but his bat makes him just an average left fielder,” they’re forgetting that DeJesus’s defense will increase his “runs prevented” over a replacement left fielder (compared to a replacement center fielder). Dave (Cameron’s) point is that the defensive gain often wipes out the offensive loss.
I haven’t sat down and figured the math myself, though it strikes me that Dave may be overstating the case. It’s still a legitimate general point, however, and one worth remembering. Thanks to guys like Dave, sabermetrics is coming along in its appreciation of fielding impact.