Why rev X.org anyway?
I mean sure it has a few bug fixes. Nothing particularly major.
On the other hand it, as people have already pointed out, breaks the proprietary drivers to rev X.org. I don't understand why they need to update it at this time even if they weren't working on Mir. Other OS's don't play whack-a-mole with their driver model every few months either, do they?
And besides, Ubuntu allows you to COMPILE AND RUN ANY CODE YOU LIKE, so if you want to update your version of X.org for whatever reason in Ubuntu 13.04, then you are free to do that. By the way, you can also remove Unity AND Mir (whenever it arrives). Shock and horror!