With Wayland, there's open development, where DE developers are welcomed to join in and contribute and write extensions and offer them to be merged to the core protocol and everyone benefits from the work of everyone. Everything is out in the open, there's no asymmetric CLA's (for those who care about such things, personally I've never seen it as a huge concern but it matters to some and thus it will alienate many developers like Upstart did) and there's no requirement to make Unity first, everything else secondary.
Everyone can take Wayland and make it work for them, because if it doesn't have something they need, they can add it, but the same is not true of Mir - Mir only cares about Unity. If you wanted to use Mir and said "I need to extend the functionality because I need this thing for my DE" Canonical might say, "sorry, we don't need that for Unity" and that's that, no dice. With Wayland, you can always extend it yourself and if the extension is good, it'll get adopted in core protocol, but even if it doesn't, you can still use it in your own DE.