Well, Canonical is running a business... they have the right to do things their way and that may mean breaking some perceived obligations to the wayland developers, after all. Business is business. I think the worry stems mostly from the fact that wayland would lose proprietary driver support and fall off, but I think it will probably work out seeing people saying that the proprietary drivers will be egl-based (whatever that is) and will probably work for both wayland and mir. No use with the hate posts now. I think Canonical will still 'support' Wayland in their official distro line-up, if they intend to keep KDE and GNOME, which I think they will. In the end, it's free software, and competition is healthy and drives innovation, or so they say, or used to say a year ago when I started using Linux.
As far as I understand, Mir is a modern type of X made from scratch, it acts as a mediator which draws stuff for it's clients/compositors i.e., it's still a display server, and Wayland is a new pardigm, it is not a server, it's a protocol that enables clients/compositors to become their own server and draw stuff themselves, interfacing directly with the display hardware. What benefits one has over the other, I do not know but they both claim to provide faster graphics with less latency than X which is apparently bloated. in the end, I think people will use whatever works out the best..