I suspect at some point Office will release a bunch of apps targetted at the latter group (remember Microsoft Writer for blog-writing?), or even have a dual mode where you can write basic content into Word whilst on a train, and can then flip back to Desktop mode to finalize it. I don't think there's any chance of them moving to Metro-mode wholesale in the near future.
The suggestion that you'd be fighting excel on a tablet screen in Metro-mode sounds dreadful, but that doesn't mean that there isn't a middle ground where some parts of Office move to Metro, other parts stay in Desktop and some apps (like Outlook) sit somewhere in the middle.
The duality approach definitely has some promise but I can't help but think of the Control Panel/PC Settings dilemma. Microsoft doesn't seem to concerned to provide a nice, unified experience. Now with an Office app there wouldn't be so much of a division of features but I would be concerned that the Windows 8 Store App version may get new features and be better maintained than the desktop version. IMO, they really need to stay in parity as much as possible. Similarly I'd be concerned that Microsoft commitment to the desktop version would lesson with the typical "we only have so many resources" excuse. That aside I agree as easy toggle between the two modes (desktop and Windows 8 Store App) would be a big hit.
I'm willing to bet that they take the same tact as they are presumably doing with Office Mobile for iOS and Android. I base that off of what they've done with OneNote on those platforms to-date but it's still way too early to tell where they are going to land the plane. Hopefully they will provide a much more feature rich and easy to use mobile suite than their existing competitors on iOS and Android. IMO they kicked Google Doc's arse with the Office web edition. Here's hoping for the same on mobile some day.