GNOME Shell is the shell of GNOME 3 and thus depending on the GNOME Shell from the gnome-session from GNOME 3 is not artificial but rather a requirement. The fallback mode is intended as a fallback for driver and VM problems, not as first-class desktop environment. You will be allowed to force it to use the fallback mode should the detection fail to understand your hardware but that's not quite there yet. See here:
http://mail.gnome.org/archives/desktop-devel-list/2011-January/msg00008.htmlhttp://mail.gnome.org/archives/desktop-devel-list/2011-January/msg00138.html
So, doing what you describe from the UI makes a lot more sense (and explaining why) from a user perspective than, "If you drivers don't work do this magic with the package manager." The former is helpful; the later is pain.