There's the psychological nature of Microsoft "buying off" exclusives (I'll be using the term loosely): by buying exclusivity from publishers, it gives PS3 owners the feeling that the publishers are treating the PS3 as "second rate" or have abandoned them.
There's also the economic one: in these trying times, it's easier to buy off exclusives than maintain studios creating one. There are lull times between projects, as well as long conceptual pre-production-- you have to maintain the development team during these times. Some have considered Hollywood-style per project contracts on the devs. But still, if a company has money (like Microsoft), they can look at currently developed projects and pick one they think will be a hit (or supports their marketing strategy).








