Developers at Canonical have concluded that the Mir desktop server (or, more specifically, the XMir layer) will not be ready in time to be shipped as the default configuration in the 13.10 release — though they do still plan to go with Mir for Ubuntu Touch. "More specifically, the multi-monitor support in XMir is working, but not to the extent we'd like to see it for all of our users. The core of Mir is working reliable, but with XMir being a key component for our 13.10 goals, we didn't want to compromise overall Ubuntu quality by shipping it."
October 2, 2013
No Mir by Default in Ubuntu 13.10
Read more at LWN