Or .. we can do this revolutionary thing and realize that a person sitting at a desk producing a proposal, sending documents to a printer or two, embedding objects between Excel and Powerpoint, and pulling in live data is a different audience than a guy sitting on a train writing a blog post and needs more than four functional features, thus necessitating a different feature set, and probably different UI options.
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.