As you may know, Ubuntu 13.10 Saucy Salamander will ship with Mir by default and XMir replacing the old X.org. While Kubuntu 13.10 refuses to switch to Mir, in order to use Wayland, the Lubuntu team has also announced on their official site that they will keep the old X.org server , until Lubuntu 14.04 is released. (more information can be found in the mailing list)
Lubuntu avoids Mir and XMir as much as they can, since the Ubuntu support for X server will stop, with the release of Ubuntu 14.04 LTS.
Those who want to test XMir on Lubuntu 13.10 will be able to easily install it easily, because it will have its own PPA.
Lubuntu’s refusal for XMir is legit, since the OS has better performance with X. And this really matters, since Lubuntu is popular for being lightweight.