A wheelset is a complete set of wheels: hubs, spokes, rims, tires, and gears.

I have two wheelsets since realistically speaking, I will infrequently change between slicks and knobby tires on a single wheelset. I would say, "It will take me 5-10 minutes to change tires..nah, I'll just ride"

Others have mentioned what it is like to ride knobbies on the road (slow, loud), but also don't forget that knobbies wear more quickly than a set of slick tires. Wearing out a $45 knobby on roads is a bad idea when a slick cost $20.

'Guin