To answer your questions (apparently I forgot to do that ):
1) The dialog allows you to load roles on demand, rather than load one on startup from the users profile.
2) Currently there is no way to see what changes a role will make to your ribbon when loaded.
As @ScottFelber stated, you stand to miss out on new functionality if you load a role from an older version. Over 100 enhancements went into NX 11 alone, so there's something to be said about recreating your roles for each new version (though I realize this is time consuming). I suspect your solution of running concurrent versions and comparing tabs is likely going to be the quickest.