I think the vast majority of games are always going to exclusive and there will always be some games that nearly have to be multi-platform.
Exclusives, as others have said, do indeed define a particular system. When someone says "Nintendo" you think Zelda, Mario, Metroid. When someone says "XBox" you think FPS and Halo. When someone says "Playstation" you think Final Fantasy, Suikoden, God of War, Ratchet and Clank, Jak and Daxter, etc... There is nothing wrong with this. Each console has it's strengths and weaknesses, and developers should play to this to give gamers better games.
On the other side of the coin, games like Madden and license games have to be multi-platform in my opinion. Sports games and Movie games go multi-platform to maximize their profit. Honestly, EA says "Madden '09 will be PS3 exclusive." Who's going to go buy a PS3 just for a football game? (Outside a sports fanatic.) But you say "Halo 3 will be XBox 360 only" and you'll have a lot of people that will buy the system just for that 1 game and maybe a couple of minor ones.
Some developers are going multi-platform with their products to try to get more of their product into households across the globe. For the most part, how many of those games are truely "generation defining?" Think on it for a moment. A vast majority of your cross platform games are good games, but they're not great games. To a point that hurts us, the gamers, but at the same time I don't really think so.







