Blind to everything except his/her/its own fanatical worship of all things shuttleworthy.
Both ubudu and gonm fanboys are horrible
and to stay on topic GTK+ is dead for me because of
- their hypocrisy
- glib usage (gobject hell, useless data types, it claims to be portable but in reallity isn't, abort on g_malloc failure, see http://suckless.org/sucks)
- UGLY (both on code and on the appearance of the program), uglier than YOU. Can you see how horrible it is?
- more thing that I'm bored to write
As for Gnome's decisions: What did you expect? Canonical made the choice to go against the community instead of working together with other Linux distros. That's why this whole Mir idiocy was such a bad idea. I personally have nothing against Ubuntu or Canonical (apart from the arrogant, rude and downright insulting behaviour of the leader of the company) or users of Ubuntu, but I, like many others, could foresee when this Mir debacle started how things would develop. There would be no support from Gnome or KDE. Of course not! Those projects are invested in Wayland, have been working to port their projects to Wayland for ages, and then some new NIH thing from Canonical with countless questionable aspects (the monolithic control, lack of standards and protocols, lack of collaboration with the non-Ubuntu communities, spreading of FUD and lies with announcement) comes along and demands support from them. Again, what did you expect?
The OS X backend is, like the Windows backend, quite a different thing - those backends have been developed way before this thing, as a means of providing cross-platform support - many software developers want their software to run on other OS'es, to gain a bigger user base, and also maybe partly because it's a good way to initiate non-Linux users into the wonderful world of FOSS software. Those backends serve a purpose.
Mir however is entirely unnecessary. It's not a notable platform. It's problematic on many levels. Suppose I wrote my own display server with incompatible API's tomorrow, can I then rail against Gnome, KDE etc. for not supporting it when they support OS X? Does the fact that a toolkit has support for OS X mean they are then obligated to support each and every new platform that shows up and demands support? Why do people who are not involved in a project feel they have a right to demand from a project whose developers they are not paying a cent to implement support for their pet platforms?
So it all comes down to this: if you want Mir support for GTK, go develop it yourself. Or ask Canonical to develop it. Gnome developers are in no way obligated to provide it for you.