I'm not sure Canonical supports XFCE in any way, and now they also no longer support KDE (financially, that is). There were hiring
one guy to work on Kubuntu, that's not much if you ask me. So we're down to two (Gnome-shell and Unity), and the support of gnome-shell is almost mandatory. Unity itself is based on gnome3, so they need to help maintain and improve it.
There is no harm in letting community driven efforts take place. Having a variety of desktop environments available to pick from has its advantages. For example, Xubuntu is really useful for people who need to cut down memory/CPU requirements.