I started wearing a helmet when I started cycling again as an adult and was making enough money to afford a good one.
I didn't go looking for proof that helmets work or don't. For me it was just a matter of wanting a nice buffer between my head and the pavement. Since I started wearing my helmet, I've only had one or two whacks, but both times I was glad it was the helmet contacting that asphalt.
Even at low speeds, bare head + pavement (or worse) = ouch. Besides, I don't want my customers seeing a bloody head. It's bad for business.