How could they "pull the plug"?
It's not a "stupid beef" It's smart long term thinking. MS could pull the plug on the Steambox any time they wanted to. Or at least make things very difficult for Valve.
If it's coming out this year then I think it needs more than the nonexistent current support instead of the magical "future support", it's just not very encouraging at all.
Steam Linux is still in beta; everything HIB should be on there for example, and I can't see any problems with the OSX titles being ported too.
If this is being treated as a new platform however, it's not what is currently available that is important, it is future support.
I dunno, most PC users have a bunch of other crap running in the background while they game. I agree that a reference model level of hardware would be a good thing though.