the toes are the fulcrum, your body is the lever. a slow speed amplified by 6 foot of lever gets the head moving pretty quickly. you don't always fall to flat ground. a curb, a fire hydrant, a retaining wall, could be in just the wrong place. the hand you thought would break your fall and keep your cranium from striking the pavement may get stuck in that pocket searching for the energy bar.
sure all those things could happen just walking down the street. but riding exposes you to those walking risks at much higher frequency. enough to warrant the helmet. as an ex observed trials rider, i know how quickly things can happen you never expected.
i used to not wear one because of helmet hair. i'm retired now and who gives a rats @$$? i even found one that provides better protection for the back of the head where the part of the brain is that controls BREATHING.
my 2 sense (haw) dabbindan