This game is very important to Microsoft for several reasons. It's one of their best chances of gaining even more converts from the PS2 and it's a game that has a lot of appeal in Europe, a market in which they would like to see the same success that they're having in North America.
If the Halo 3 level of advertising it to be believed and if it's coupled with a price cut it could help Microsoft to cement it's position in second place for several years if not this entire console generation.