How is this really different from what exists today? You can write in C# for pretty much every MS platform already. If you're smart you're already writing your business logic with as little dependencies as possible on view specific logic so it's more testable, portable, etc.
Not sure if you have tried doing this. In practice, it's quite painful as you need to have a separate visual studio project file for each platform (phone vs tablet).
Further, the Phone implementation of WinRT is a subset of the tablet's, so sticking to the WinRT API alone isn't enough. Complicating matters further, the phone lets you actually use a Win32 subset which is not allowed on tablet.
This (appears to atleast) unify everything towards more of a write once, run on all windows platforms world.
