Everyone her that uses it loves it. Why does MS care if gamers buy it?
It's all about linking everything together, all devices, services, games. etc. and OS's are Microsoft's strong suit and solid base to build that foundation. There's a book called Xbox Uncloaked that came out a few years ago that explained it. Microsoft mainly got into the gaming console race because they knew Sony was trying to bring services normally found on PC's into the console market such as web-surfing, IM'ing shopping and such which is a direct threat to PC's. They first tried to build on their relationship with Sega but Sega wasn't interested so they went it alone. They were thinking long term here so they knew 6-7 years ago that whatever the second machine they made would be, it would be an extension of both the PC world and that of living room entertainment.
Of course here we are years later and they were right on the ball, set top TV web boxes, DVR's, handheld media players, shopping on the Wii and getting weather and news, linux on the PS2 and PS3. So building Vista around the philosophy that they could bridge the gap between devices with it was one thing, they still needed gamer's with 360's to want it so here we are with Halo 2 on vista.
After that comes Forza which will use phones/zune's 360's and PC's for customizable cars and tweaking and be able to swap data between all of them and then Shadowrun which is the first ground up game to use 360/PC connectivity as was intended.
So of course a gamer with devices that are linked in such a fashion will most likely buy Microsoft branded apps, and games which in turn will be Microsoft OS specific, and in turn keeps Microsoft's most important cash maker Windows on top of things because the PC remains the middleman between all these devices.