If Valve makes a complete custom environment for steam OS built around big picture will they really benefit more from using ubuntu versus debian, fedora or any other distro?
Does anyone _not_ expect Valve to throw that all away in favor of big picture mode? Unity, if it does ship in Steam OS by default, will be the under-the-hood interface.
But even if they are working with Ubuntu for some of the reasons you mentioned, that doesn't mean that they have to go with Mir. You could install Mir for any distribution. You could install Wayland on any distribution. You could just keep going with X because at least it's stable and people know about it.
I personally switched from Ubuntu back to Debian when I realized that I was dealing with Ubuntu-specific bugs more often than I was benefiting from Ubuntu-specific features.