As you already know, Canonical will replace the X.org server with Mir, on Ubuntu 13.10 Saucy Salamander. This means that the developers have to finish the work on Mir until October, when Ubuntu Saucy will be officially released.
Many graphics card developers consider Mir immature and refused to bring Mir support for their hardware. For example, Intel has dropped the Mir support in order to invest in Wayland, the default system compositor for Tizen, Lubuntu and many others.
Canonical does not worry about Intel decision to drop the support for their compositor, since they are open source, the Ubuntu developers themselves can optimize Mir for the Intel hardware.