For all of the polotics and Mir bashing, it was the right way to go for them. Since they are running Compiz/Unity8 they might as well finish off the stack. Mir makes their life easier for Compiz and Unity 8.
While it may not benefit the Linux community with code, they are currently one of the top distro's. I am not much of a programmer, but I imagine that if you write an application for Ubuntu using their SDK (based on a variant of Qt) then it wouldn't be much to port that to another Linux distro. That should help the community a great deal. The OS's are great, but application scarcity/popularity are what keeps people from using it.
The other major issue for Linux is drivers. While Mir is a mess with the video drivers, and things have gotten better, device manufactures still don't target Linux as a major platform. If they gain market share and consumers want it, then more drivers will be written, validated, and tested at launch rather than a months down the road.
I do wish Ubuntu would have stuck with the stuff everyone else is using. But at least they are dropping Upstart for systemd. Canonical may be polarizing and not the ideal company to gain marketshare, but I remain optimistic.