The Ubuntu Unity desktop currently relies upon the GNOME 3's "fallback" code for items like the Bluetooth indicator, Keyboard indicator, and the GNOME fallback session code is also used for dealing with disk mounting in Unity.
Upstream GNOME is planning to drop the GNOME Fallback support eventually, so Ubuntu developers will be looking to stop relying upon the code-base. This GNOME code would then be replaced with their own implementations geared for Unity. GNOME developers eventually just want to push the GNOME Shell and not maintain the non-GL-dependent GNOME Fallback/Classic mode and when a system without GPU driver support is there, the user will be forced to use LLVMpipe -- something that's now happened in Ubuntu 12.10 for the Unity desktop as Unity 2D was dropped.
On the plus side, in rewriting the code for Unity the new implementation might be better. "If we rewrite them we can as well do it in away which improves the design."
This proposed discussion for UDS-R in Copenhagen was brought up a number of days ago on the ubuntu-desktop list in "Stop relying on GNOME fallback code for unity."