Every distro other than Ubuntu is going to use Wayland. Every other desktop environment has plans to move to Wayland. Wayland will be used on many mobile and embedded Linux systems. It is something that has the chance to bring the entire Linux ecosystem together under one graphics standard. Every major toolkit will support Wayland.
By contrast, Mir is a fixed implementation - there is no "Mir standard", you can't develop your own Mir server because Canonical can just break the API. Mir is designed only for the needs of Unity, it does not take in account the needs of any other desktop or distribution. It will not benefit anyone other than Canonical. None of the major desktop environments or toolkits have plans to implement Mir support.
IMO, reimplementing the drivers should be a matter of choice ("do I want to use this closed source driver, or rather use open ones?"), not a matter of need because the infrastructure I use is incompatible.
There is one other advantage I'm not sure if is possible with Wayland (but I'm not sure if it's implemented on Mir either) that is the use of a smartphone as a thin client. I wonder how do they expect to do that.
Advantages over X.org were discussed since day zero until today. What are the advantages of Mir over Wayland?
All of this happens EVEN IF Canonical do the work of maintaining. And except they put a full-time dev on it, this will generate breakages constantly, or a massive slow down (if they do wait till Canonical tests it still works to keep committing). Forget, for example, using the latest version on Ubuntu, since it will only work with the official packages which will involve first fixing again Mir backend which will be for sure broken for untested upstream changes.
There is no technical reason that Mir is needed, just like there is no technical reason for gtk or gnome or even Linux itself. The reasons they chose not to use Wayland are here.
And most do apply to Mir. The thing is, the charge of the proof is for the one proposing the change. The question "why Wayland?" was answered before Mir was even an idea. The new question is "why Mir?" and I didn't see any technical answer yet, except from the two I already mentioned on my post, that does not apply in EXACTLY the same way in Wayland. And even of those, one has no technical details of how it's done (or planned to be, if it isn't implemented yet), being that the reason I can't tell 'well, Wayland does the same', and the other would just require using a compositor abstracting the platform.