I think the biggest reason why timed exclusives is because they have been successful Bioshock, Lost Planet, Ninja Gaiden all have out preformed Sony. With all but Ninja Gaiden 2 doing better than 2-1 sales. The reason why this is important for Microsoft is often multiple console owners will pick up a copy on 360 rather than wait for the PS3 version. Second, when the 9 months or what ever is up there are a lot of new games out that command money from PS3 gamers so either buy a game that may or may not have been awesome months ago or buy something new.