Matt Diaz and the right-handed platoon

The Pirates signed Matt Diaz to be a platoon player. At first, the signing sent me off on a bit of a chicken-or-the-egg train: Can 105-loss teams have the luxury of platoon players? Or would 105-loss teams not be 105-loss teams if they knew how to effectively employ a platoon? That's clearly an unproductive line of thinking, but then I started to wonder about right-handed platoon players. How many right-handed platoon players are there? The value of a left-handed platoon player is obvious. Garrett Jones, who Diaz will be sharing time with, had 654 PAs in 2010 and 424 of them were against righties. A right-handed platoon player, though, is a pinch-hitter two of every three nights. This may just be my own bias showing, but when I think of platoon players I think of lefties that can't hit lefties at all and not of the righties that tend to replace them a couple nights a week. That leaves me with a couple of questions: First of all, can a player be effective only playing against...