I've seriously damaged three different helmets in three different bike accidents by hitting my head hard on the ground. I would have been hurt pretty badly in all of the accidents if not for the helmets.

I have a 5 mile (each way) commute and I wear my helmet every time. For leisurely trips of a quarter mile or so, I only sometimes wear a helmet.

I do ride my bike at speeds of up to (i'd guess) 30mph if the wind and hills are favorable, but for any speed beyond what I could run I consider a helmet necessary.