Mir is irrelevant. No one heard of this a week ago and plans do not change based on wild speculation. If Ubuntu is not clear about what Ubuntu wants it to be, it is your problem. It has nothing to do with Linux Mint.
This is what Clem said when Swapnil Bhartiya in an interview for Muktware asked him about how Mint will handle the transition from X to Mir in future Ubuntu editions. Clem also stated in that interview that what Mint cares about is not the base distribution or the desktop environment, but the final product and the experience given to the user, he denied that there were plans for Mint to abandon the Ubuntu base and build entirely on Debian and denied working on tablet versions of Mint saying their approach is to make Linux Mint 15 (which will be based on Ubuntu 13.04) better than 14.
The full interview here
http://www.muktware.com/5356/clement-lefebvre-mir-irrelevant-linux-mint