Xubuntu was the first distribution to provide XMir-based images
Xubuntu developers have decided against the integration of XMir in the next 13.10 release, even if they were actually the first to release XMir-based images of their distribution.Canonical will implemented Mir as the default display server in Ubuntu 13.10 (Saucy Salamander), but so far none of the official flavors based on Ubuntu has agreed to add it to their list of concerns.
A couple of weeks ago, Xubuntu developers made available a version of their operating system running on XMir. Users haven't been providing great feedback and a lot of them have complained that it affects the OS performance.
A decision has been made by the Xubuntu developers to not integrate XMir in Ubuntu 13.10, and the same goes for Mir.
XMir is an implementation of X running on Mir. It's designed to work as a buffer for the developers who don't want to go straight to Mir.