Cheap (no initial cost of buying shifty bits, cassettes and double rings)
Reliable (does not come out of adjustment, does not freeze into the wrong gear in the winter time or with mud)
Builds strength at different cadences (similar to a fixed)
The mechanical simplicity is attractive to some.

I have to disagree with you. The only "advantage" you trade off is that you can't skid, and you gain the advantage that you can coast down hills or when traction is sketchy. You can also corner sharper without worrying about pedal strike.