Mark Shuttleworth shared on Google+ a post by Christopher Halse Rogers about the Canonical motives for working on Mir. Interestingly, Mark claims, "Contrary to competitor FUD, from people who are not working as openly as they would have you believe, Mir is more likely to enable high-quality graphics for ALL flavours of Ubuntu, and any distro that chooses it. Graphics vendors have been happy to engage and ensure it works well on all architectures."
Mark additionally notes, "Delighted to give the Mir team my full support. It's classy work that will make all *buntu's sleeker, faster and more responsive. A great contribution to your open source options."
Rogers, who in recent years has worked on X.Org packaging for Ubuntu at Canonical, basically summarized the Canonical motives for working on Mir rather than Wayland is that at the time they figured they would need to write an input stack, patch the Mesa EGL platform for server-side buffer allocation, and redo the window manager handling in all of the supported tool-kits.
This is based upon their belief that they would have to likely fork the Weston compositor, at the time the Wayland input code wasn't in a pristine state (since then it's been much-improved thanks to work by Daniel Stone and others), ARM hardware liking server-side buffer allocation (it's supported by Wayland though, just right now Mesa isn't using it), and that they want to minimize possible complexities in the stack.
The Canonical employee ended his Google+ posting with, "Mir is a Wayland-alike; we're piggybacking on a lot of good work done for Wayland. Hopefully we'll contribute back not just an awesome display server in the form of Mir and an awesome desktop environment in the form of Unity, but also low-level improvements that can be used by Wayland compositors. I'm particularly excited about our engagements with NVIDIA and AMD; although it's early days, I'm hopeful we can get a solution for “but what about proprietary drivers?” not just for Mir, but for everyone."
He had another Google+ post this morning with more Mir talk. In short he says that Mir doesn't break anything that works now since non-Unity desktops can still use an X.Org Server, a root-less X.Org Server will be available to run X11 applications on Mir (just like XWayland on Wayland), and that no applications/desktops are yet to be Wayland-specific for causing potential problems with Mir.