It's not up to Microsoft whether or not PC can play with 360, it's up to the developers. The ability is there (see Shadowrun and Lost Planet: Colonies Edition) but developers just aren't taking advantage of it.
Hell, my Steam version of Fallout 3 has a tab for LIVE, and when I clicked it, it made me create a profile. As I type this it's trying to download a LIVE update. From within Fallout 3. From within Steam. There are so many ways to distribute a game and get it, even just counting the legal ways, that treating LIVE as the one stop for all your multiplayer needs just doesn't work.
In the PC environment you can't force every developer to add LIVE functionality or achievement bullshit. Steam understands that, which is why it's superior.
They both have their own damn niches.
Steam works as a PC platform because it's tailored to PC. I don't see it working well on a 360.
Note i said "match making "since Xbox live has no fucking damn real servers, for 99% of the games only a bunch of server that pair you up with another machine that is not a service.
In the PC world you get real powered servers on real connect free not paired up with some lame ass console boy with his dumb console on a sucky house connection.
Of course that pay for online pay died real fast after 3 games and nobody has gave jack since you can get real better service for free on PC and always have been.
So I was pleasantly surprised to see how seamlessly I was able to sync with my XBox Live account. I could see the status of my online friends, send and receive messages, etc. Probably the best thing though, is achievement support. I think it's great that my achievements and gamerscore are reflected across both platforms (though I think it's kind of dumb that Fallout PC is considered a "different game" than the 360 version, as far as doing achievement comparisons and such.)
So really, I think the benefit is determined by where your "gaming community" is centralized. Since all my gaming friends have centralized around XBox live, it's nice to be able to still be plugged in, even when I've opted for the PC version of a game.
What needs to happen now is more cross platform multiplayer. I mean, Left 4 Dead is a great example. All of my friends will be getting the 360 version. 4 player co-op, being central to the game, is pretty much dictating that I also get the 360 version, despite the fact that there are a hundred reasons I would really prefer the PC version. But I want to play with my friends, not by myself, or some strangers. The positive experience of blasting zombies with buddies easily outweighs the negatives of being hampered by console controls, etc.

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