Whether you like it or not, power users are probably the only audience left for a linux DE that's no
longer the default on the most newbie friendly distro's. The rest of the world is never going to see it.
Open source is very much about scratching one's own itch
If Unity is described as a Shell for the GNOME desktop environment. Then that sounds about right to me.
Perhaps better questions to ask would be what is Gnome 3? What is Gnome Shell? And what is Gnome? Sorry but it's a bit difficult to comment on Gnome when it's not entirely clear as to what it is.
Yes and no. They ship with a lot of gnome3 apps, but none of the unity code uses any gnome 3 libs. They do use compiz though, and have compiz use the gconf settings.
The thing is, they could switch to a qt based desktop, without touching any unity code.
Open source is very much about scratching one's own itch