Right now there are a bunch of little forked differences between them all. Microsoft says they are working to merge all the shell differences into a single project (CShell; composable shell), and the point of getting to a clean core (the "Polaris" codename in this article) seems to be to make sure all the rest of Windows modularly slots in on top of that, and a benefit to that is that you could quite possibly more easily change between which modules are loaded/available.

From the Continuum project, we know that Microsoft finds it interesting to be able to switch between Mobile and Desktop editions as much "on the fly" as possible. Could mean they have ideas for switching between Desktop and Xbox modes, too.