In Safeco, Lefties are the best production so we stock up on them. Righties die.
But there is no upper limit on hitting the way there is on fielding. If you have 3 guys hit 40 HR each they aren't stealing HRs from each other.
Once the defensive-chance spheres overlap to a significant degree you're just wasting space out there. Cameron used to run all the way to where the LF was standing, basically, to make plays. The fact that he was given the CHANCE to do that doesn't mean that the play would not have been made without his presence on the roster. The LF would have had to do more, but it was within his ability to do.
If Ichiro is in RF and Upton is in LF, and neither guy is exactly an immovable tree-stump with an oil can for a glove, just how important is CF defense? If the CF is always calling off the other fielders who could have made plays just as easily on the ball, then the glove-whiz CF is not necessary for those plays. A lesser CFer could have done just fine. So the question is how much overlap is there out there on Defense, and how much is necessary to maintain pitcher ERA?
I agree that OF defense is important, but again, I'm not trying to replace Saunders with Justin Smoak when I move Mike to CF. Nor am I trying to replace Gutierrez with Raul Ibanez in center.
Fielding is not hitting. The point of diminishing returns in the fielding game is real. It's not a 1-to-1 add like offensive production. 3 Franklin Gutierrezes out there wouldn't be able to fully put their skills to use. It wouldn't be necessary for the vast majority of the plays. On most plays, any competent defender can get there. And the overlap in their field coverage would make the defense a secure blanket, but not outrageously more than Upton + Saunders + Ichiro, who would all be plus defenders at their positions.
At least, I don't think it would be outrageously more, and not enough to compensate for the loss of offense.
This is my problem with defensive analysis: You can't just take 20 runs off for offense, add them back in on defense, and call it good. There are upper limits of maximum effectiveness with gloves. There are hits that Guti gets to that no other defender would, but the vast majority of his work can be done by any competent glove. If Saunders is a butcher in CF, this is a problem. If Saunders is Winn, then it's far less of one.
And the extra 30 OPS+ points from Upton would help salve my wounds on the relatively few balls that Guti would have caught that Saunders could not get to.
~G