(Posting from cellphone so terse)
Paul Thurrott has a useful consideration on Metro v Desktop in a column
here.
I'm pretty confident that the plan all along is to have Desktop for business/power/legacy users, and Metro for handheld touch devices. MSFT doesn't expect us to give up Desktop for "Fisher-Price" Office and productivity apps, but it knows Desktop won't cut it on tablets. There are millions and millions like me who mostly live in a power desktop and who also want a no-compromise tablet for easy browsing, etc but also want the tablet to drop back into serious apps when required. Having a single OS that does both, with all the x-platform compatibility, services (such as comms, multimedia and manageability) - and above all common apps - is the best of all worlds.