If you are talking about pushing console to the limits graphically then exclusives usually show the true potential of the console. This is because in multiplatform games developers try to make different versions look alike (at least when we are talking about PS3/360 games). You can't create a game which looks exactly same on both PS3/360 and still reach both consoles limits, there are too many hardware differences between PS3 and 360. If you would allow versions look a bit different then they could also reach limits with both consoles but that might not always be a good idea financially because you would have a lot of complaining customers. With exclusive games you don't need to worry about that.
Then again, graphics aren't the most important part of the game anyway. Gameplay is always much more important and there it doesn't matter whether game is exclusive or not. Best game of the console can just as easily be a multiplatform game.
