What I think both gnome-shell and the metro interface attempt to do is put the task you're involved in at the center of the experience.
But their attempts aren't successful in a lot of cases because a task may use more than one application, and keeping both applications visible provides context to the user. One example is reading a web page and taking notes. This task uses two applications: a web browser and a text editor. Another example is web development: a text editor in one or more window to edit HTML markup, style, script, or a PHP program, and a web browser in another window to view the output. The task involves refreshing the page, inspecting the output, and tweaking the code to make the output more closely resemble the expected output. This task likewise uses more than one application.
Yes -- you're right of course. If an application can't contain your task, then the metro-app=task equation fails. This is where gnome-shell, with its multiple desktops, succeeds. The shell focuses you by stripping away the distraction of panels, etc. but also allows you to set up tasks however you want by using multiple desktops and making it easy to snap windows into place for multi-window use. The way gnome-shell allows the dynamic creation and destruction of desktops also encourages you to think of each desktop as a separate task.
It wouldn't be hard for Windows to follow a similar model, but it seems like they don't really conceive of their desktop as a part of their metro interface at all -- it's just an ugly 2nd cousin of the new interface.
But I imagine we exaggerate the importance of looking at multiple windows at once.
I'd say any task in which you traditionally would have overlapping windows rather than side-by-side windows would be a case where switching between full screen views wouldn't be so bad. The utility of the overlapping windows is to give you an easy way to remember what else you were doing and how to get back to it -- the metaphorical visual "stack" of windows mirroring the "stack" of stuff you're doing -- and both Windows 8 and gnome-shell have fast ways to switch between tasks you were recently doing (getting to your "last" task is especially fast w/ touch on Windows 8).
Finally, I'd add that on most laptops, side-by-side screens aren't really that great: a typical laptop at 1280 or 1360 pixels wide doesn't really allow two standard webpages or word documents to be displayed next to each other, so unless your work involves a terminal or a text editor, it's unlikely the side-by-side windows are all that handy.
Of course, all of this will be different if we can get really big displays -- those will call for whole new UIs, and in those cases, we'll be able to keep lots of windows spread out much more easily -- but for now 1920x1080 is the biggest most of us will be using and even at that resolution, two full applications is the most you'd ever really use side-by-side, and that is the one use case already built into the "metro" interface on windows 8 (albeit with flaws, mentioned earlier, but flaws that presumably could be improved in the next upgrade, much as Vista was fixed with 7).