It is at present, and I expect it will be possible to use other desktops for 14.04, which is important as it is a Long Term Support release with support for 5 years.

I'm a bit hazy about what happens when Mir replaces X. I gather that this particular argument is about ensuring that the compositor for KDE will work with Mir, or, more accurately, who does the work to ensure that KWin will work on Mir.