No, you don't need a rack, necessarily: if there's no need to carry anything, or if you don't mind carrying everything in a backpack/bag I guess.

When I think commuting, I think utilitarian. For commuting, my bike is my beast of burdon and the rack is where I put all my ****. Makes more sense to me than putting it on my back. My commute can be pretty long and the weather here is pretty crazy, so I also tend to keep extra clothes, and lights and stuff like that on the bike so it's always there and so i don't have to think to bring it.

Anyway, the ultimate (commuter) bike, for me needs a rack.