Gnome 3.8 is dropping Fallback Mode
Drop or Fix Fallback Mode was the first feature under discussion for 3.8 release. The first thought for removing Fallback was in Gnome 3.6, that didn’t happen but now is closer than ever.
After all Fallback Mode was a hybrid solution for hardware that hadn’t 3d-acceleration to run Gnome Shell. Gnome Shell since version 3.6 can run with software rendering, so it was matter of time to drop it. However this decision has some negative side-effects to other software like Unity.