They're probably anticipating the downfall and mad rush away from Windows 8
WinME was an unmitigated train wreck. The numbers for Linux didn't improve. Windows Vista wasn't much better. And had the same results. Considering that the largest issue that anyone can talk about is the Metro UI, I don't see that the same results won't be forthcoming.
What MS seems to be doing (to me) is to not really work on a Windows 8 UI. They're working on a Windows 8 Kernel (including ARM support), but a Tablet/Phone shell. What they could really do to salvage the PR disaster that's bearing down on them is to be forthcoming with this, i.e. Metro = iOS, and quit referring to the standard interface as the 'Classic Shell', but rather the 'Desktop Shell' or something similar. Default to the 'Desktop Shell' on the desktop, and the Metro Shell on devices its meant for. But leave the ability to switch between- which gives them a leg up on the whole iOS/MacOS dichotomy that Apple has created.
Just my opinion, of course. I'm upgrading to 7 on my XP machines right now, so I know I'll be skipping for a while in any case, as I think I'll be looking at Clojure before delving into WinRT unless forced.
AFAIK, in a stable system (nudge, wink) as long as the binary is compiled correctly and calls the right versions of supporting libraries (which can be statically linked, if need be) then it'll work just fine.
I have, in times of desperation, used Red Hat and Slackware binaries on my system when Debian didn't have what I needed, without serious repercussion.
So, it's very possible, but one can fairly reliably predict a new Steam/Valve forum section dedicated to posts like "[game] isn't working on [Linux distro]"...
Stable code base is quite a different thing from stable system. Every version of the source engine is not the same. They've finished and cleaned up the L4D2 version of the source engine, that would seem the logical choice to use, especially if they're still currently working on the most recent version of the source engine.