I think what confuses me about the GGP is that at some point don't you just get both a desktop and a laptop rather than trying to turn a super-laptop into an on-par desktop?
But the key for me is I do that because I have a work laptop and a personal desktop and I want no question about the ownership of things I do on the desktop.
I organized my windows and tabs and splits in such a way that it degrades gracefully when I switch from the external monitor and back.
How often often do I really need that? Not often, but when I do it's really useful.