I'cve never used disk brakes, so I don't know squat about rotors, but swapping out my heavy-duty thrash wheels for my lighter speed wheels isn't much of a chore. For one thing, I have knobbies on the thrash wheels and semi-slicks on the light wheels, for another, it takes 20 seconds with a chain whip and a lockring tool to move the cassette from one freehub to the other. Not a big deal...
Okay... I have to take two minutes to adjust the V-brakes for the narrower/wider rim...