I got a spare wheelset for my Nokians, because changing tires is a pain and leads to flats (at least when I do it), but also because the Nokians are 26x2.25 or so and my slicks are 26x1.2, and rims that fit one don't really fit the other. No problems with the cassette or derailleur, but rim brakes need to be adjusted because of the dramatic difference in wheel width. So I got disc brakes.

I think my next purchase will be a snow bike.