Whenever something with over a decade of polish behind it gets replaced with something fundamentally different, I don't expect the replacement to be overall better instantly. Certainly not better across the board. That would be unrealistic.
For example,
this was a much, much smaller leap UI-wise than classic to Metro is, had serious issues at first and almost certainly wasn't as usable as its predecessor for the bulk of users, but I don't think anyone would argue it was a bad decision to make the jump.
I would be very disappointed if Microsoft stopped here and froze Metro in place instead of improving it with stuff like increasing the amount of apps that may be shown simultaneously, and properly supporting multiple displays.
But do you expect a level of functionality beyond what Metro may offer in a year or two from now?
My point is: the sum total of the functionality you see right now, or at release of Windows 8 for that matter, is not a reasonable yardstick to judge Microsoft's decision to release Metro at this point in time. If you have concrete reasons to think Metro can't/won't gain more power and surpass classic in usability, that's another matter. It's precisely because Metro is still in infancy that the classic desktop has to stay in the OS for now - if Microsoft thought Metro was ready and app support was ready, they would give classic the boot. It's obvious that the two do not fit well together.