Ed Bott blogs on Why there’s no Office for Metro (yet).
That’s the reason some of the built-in apps in Windows 8 are scrambling to add features now (Mail and Music, most notably). You can’t build full-featured robust apps until the underlying platform is solid.
The obvious question is then “why not wait until the platform is stable before releasing the SDK and waiting for devs to use it to create the new system and OS?” Isn’t it a bit of a black eye for Microsoft that on Windows RT they have to have the weird add on of the Windows Desktop that you can’t run anything but Office on? Is the obvious answer “they had to release now to stop Apple and Google from getting another half-year ahead.”?
Personally I think that the Windows RT that has a desktop that’s not a desktop but runs some things is potentially a huge cause of confusion for “normal” people. Windows RT that’s pure Metro Windows 8 App Store Style (or WTF they are telling people to call it), running only App Store apps, no desktop, and a Metro-ized office (and enough apps on day one to show you gave your developers the SDK a few months before release) would be a huge win. This weird two headed monster, no matter how nice the Metro environment and hardware is, will not be the winner it could be, in my opinion.