My oldest rides a fixie and does not wear a helmet. I reminded him of his Mom rolling her bike to avoid a cat and all that happened to her was that she snapped her collar bone. Her head was OK. Then he reminded me that helmets don't always prevent problems. Apparently on Monday in Ottawa (Ontario) a woman was bicycling and sverved to avoid running into someone who was opening their car door. In swerving she was run over by another passing car. DOA of internal injuries. Sad but it does happen.
It is still better to wear a helmet than suffer head injuries while living