Another thing to consider: As many PC and console gamers know, if companies host their own master servers for organizing games, when a new edition comes out (such as sports games) or an old game becomes unpopular, master servers are shut down, and no one gets to play anymore (unless the game is modded by the community, or another company takes over the master server).
Here, MS *IS* the master server for every title. There (in theory) should be absolutely no risk to online gamers who prefer older versions to newer. Like Madden 08 vs 09? You use Xbox Live, you aren't forced to buy 09 because EA decided to shut down the master servers. This, of course, is a nice incentive to developers too.