Its just based on the game. Games like GTAIV and Fallout 3 sold more because they went multiplatform. In fact, those games even sold better on the XBOX360, the 'less graphical' platform. So you can't deny that some games benefit from going multiplatform.
However Final Fantasy XIII is a special case, because it was heavily publicized for YEARS and for many of those years, it was pushed as a PS3 exclusive. Then it was announced to be coming for XBOX360, so some people suddenly had to decide which version they wanted. That doesn't help a game sell more units, it hinders it. Sure, it might have sold more units to those who weren't going to buy the PS3 just to buy the game, but there's undoubtedly others who now are not buying the game because they don't know which version to buy or lost the 'hype' in their purchase.
Then there's the case of those games where exclusivity does help the game. Aside from obvious titles such as Nintendo titles like Mario or Pokemon, let's look at another SquareEnix title. Something like Dragon Quest. Its a title people actually go out and buy physical consoles for. Such as the recent surge in DS sales when Dragon Quest IX went on sale in Japan and undoubtedly the increase in sales for Wiis when Dragon Quest X is near launch. Does exclusivity aid in some titles? No doubt. As there's no reason for this title to go multiplatform. People are already flocking TO the system of choice for the game.













