As to the RT API (yay for not-the-least-bit-confusing-naming, given that every Windows ever has been referred to as WinXX...), the benefits in doing so would be very limited:
- WinRT is the solution to a problem (terrible Win32 APIs) that doesn't exist on a blank slate like a new console (unless they choose to make it so), and significant portions of it aren't terribly relevant to a console. They can of course just use it as a contract with an entirely custom underlying implementation specific to the Xbox's OS and architecture, but then we have the next point:
- WinRT isn't supported on earlier versions of Windows, nor has MS announced any plans to do so (quite the contrary, in fact). So it wouldn't make Xbox<->PC porting any easier either; inability to support 7 and Vista would make it a non-starter in that regard.
Granted, MS isn't above doing silly things, so this being one of them doesn't mean they won't...