IE: if your DE of choice hasn't been converted to Wayland native yet, then too bad.
XMir: DE and applications can run (later application only mode will be supported). User can be running DE running natively under Mir or XMir for all DE's and applications.
IE: Users have more choices. Use a Mir native DE or not, your choice.
I see this as a HUGE advantage and great for users to assist with the post-Xorg times transitions that Wayland doesn't support. Its USE NATIVE or too bad with Wayland. There's a choice with Mir/XMir.