Personally, I prefer the succinctness and flow of just saying it's bad for gaming but good enough for a composited desktop compared to some lengthier but possibly more precise wording.
Anyway... I just hope they don't decide this is a good excuse to rip out the fallback. They can do what they want with Unity. Actually though this does seem like a reasonable decision, I had assumed Unity2D was a Unity fork (which may not be too bad to keep in sync) when in fact it's a from-scratch rewrite that behaves like the current Unity (but may be completely different internally, so keeping it in feature parity with Unity could be difficult.) So as long as llvmpipe is fast enough, good deal.
Unrelated comment, I noticed the excessive CPU usage as well, in a VirtualBox VM with either 11.10 (when it was using Unity) and 12.04 (Unity) -- I suppose it falls back to Unity2D -- the VM will just be burning up cycles while the system is doing absolutely nothing. With the fallback desktop, this CPU usage doens't occur (Kudos to whoever did the finishing touches to make the fallback go from looking awful in 11.04 to acceptable in 12.04...). I have the feeling some code might be running a polling loop instead of waiting for an interrupt, signal, or what have you with Unity.