Gnome shell effectively does desktop management (switching and zooming of desktops) at it's core. Virtual desktops however are merely an illusion created by a window manager.
Furthermore gnome shell seems to make heavy use of zooming, which in software is extremely expensive, but zooming of tiles in opengl comes almost for free.
One could come up with some mediocre half blood that would emulate this using no opengl, but that would mean the tricks like "taking a snapshot of a virtual desktop and scaling it in software". This couldn't be done fluently and can't do real-time updates giving it a very amateuristic and uncomfortable feeling. And this has to be made, tested and maintained practically forever.
The alternative (a good alternative for non-opengl system) would be how gnome 2 does it. That is: use a window manager that does simple window management and simple virtual desktops. Many window managers supply this (including the simple taskmanager and other panel features).
And you pager, menu, systray etc. are just undecorated windows that stick to the sides of your viewport. Nothing is stopping anybody from using them even if they are using the gnome shell (they can be combined if you really want to).
So it simply comes down to: Gnome 3 is shipping a opengl based default look and feel for it's desktop environment. Some people can't or don't want that. The only thing they (or some distribution for that matter) has to do is create another configuration that don't use these features and that could easily serve as a alternative default. This would probably depend on some non-main gnome applications, but frankly, hardly anybody uses the raw main gnome set of applications, they use their distributions version of them. Which ofcourse don't care if they depend on some non-main gnome package :-)