About being pissed despite both are almost the same anyway, it's because the API is different, and that means the software you write (except when targeting a toolkit, but there are cases where this is undesirable) can't run on both seamlessly, you need to make a backend for Wayland and another one for Mir. If they'd implement it as a Wayland compositor, they can expose their own API, but things written for Wayland will run on their compositor seamlessly, because they'd support also the standard protocol; if the Mir API is supposed to be used for apps, though, the incompatibility will still be there; it solves the problem if that API is meant only for Ubuntu and Unity specifics, since this could be done without messing with anyone else. In the worst case, you'd need to macro the window creation to allocate on the client or not, according to the model your compositor use (or maybe even be announced by the compositor, so you can check on runtime).
You losers are just too hung up over this and trying to blow it out of proportion. Canonical with do as it bloody well feels and no amount of ranting or raving on blogs and forums will change that.
Get over it and move on. Nothing is being taken from you.
I wish Canonical had gone with Wayland but they didn't and that's that.
I will still be able to use any other Distro with Wayland and Ubuntu with Mir. I don't see what the fucking problem is.