The Ubuntu guys decided that, in order to make Mir work, they better ship it sooner than later. And to make things worse, X drivers are not Mir drivers. Ah, and machines with nVidia/ATI will fall back to X, and the thing will use Xmir, which is… X over the unproven and incompatible Mir drivers. Or something like that.
TLDR: In order to fix the mess that is X, they somehow made a bigger mess. Their excuse is that this is necessary to run ubuntu on tablets, or mobile devices. Sorry, but my workstation that ran X in the 90s is way inferior than those devices. Sorry, but such a video subsystem already exists on Android. Find a better excuse for artificial incompatibility.
As if Unity wasn’t bad enough.