Metro should only be for tablets, and for computers that convert between tablets and laptops. Only if a device doesn’t have a keyboard and mouse should it be in Metro mode.
That makes more sense that Windows 8. Imagine those crappy convertible tablets that were sold in the pre-iPad days. Now imagine if they ran regular Windows when you had the keyboard and trackpad available and switched to Metro when you “converted” the notebook to a tablet — I think that would be a neat solution to the problem that was only created by these convertible devices.