I have a laptop with W8 on it and connected an external monitor to it. When I did, the Metro interface was only on the laptop. The monitor had the desktop.
It was very easy to navigate between the two and I found that using the Windows key was a simple way to show/hide the start screen. Just like the start menu.
Also, applications running in Metro do not "sap" power. Any app not on the screen is in stasis. This is to preserve power. Check it out by opening the task manager.
Also Metro apps are nice. They are clean, very fast, and easy to use. Yes, most of my work is in Office and other non-metro apps, but some, like mail, calendar, and contacts are just better in metro than in Outlook (except for certain functions, which are not regularly used by me)
The more I work in W8, the more I find myself liking the duality of the interface.
Give it an honest try. You might be surprised.