I'm going to argue that the real tablet UI elements developed by the guys at Canonical are the lenses (aka the dash), not the side menu. They are full screen,

I'm going to argue that the real tablet UI elements developed by the guys at Canonical are the lenses (aka the dash), not the side menu. They are full screen, display large icons, allow you to implement a lot of features (e.g., see Music/video, Documents and Shopping lenses) while using the same UI. The top bar is only important for its notification icons - the global menu is slowly going to fade out, replaced by in-app icons (see for example google chrome). I agree with you that one UI for all form factors and sizes is probably a bad idea, but I think I see where they are going for tablets. I pretty much hate Unity for the desktop, but I think for tablets it will make sense.