Numerous videos show that running a non-Metro app, such as Windows Explorer, from Metro takes you to classic Windows. A swipe gesture takes you from Metro to classic Desktop. It's plainly [yet] another layer over the Windows 7 GUI, which itself is a layer over the classic Windows GUI.
First, there is a "Metro Explorer". Second, not even in x86 is it a layer on top of the desktop UI. Its best described as two separate apps, loaded when called upon. Lastly, fact of the matter is it is yet unknown if ARM-W8 will even sport the desktop UI at all (i certainly hope so, but you never know).
Addendum: Not only is there a "Metro Explorer". Rumors have it, ARM-tablets will not run anything but Metro apps at all. To reuse your line of thought: without "non-Metro apps", there is nothing that "takes you to classic Windows". (Which doesn't make classic windows useless, but certainly "unnecessary").
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