I couldn't give a damn if the rules say to wear a helmet or not. There will be one on my and my kids heads. Last year my daughter was saved a nasty head injury thanks to her helmet. A cousin riding with a group took a spill; his helmet undoubtedly saved him a head injury or even worse at 20+ mph. And then there is the boy who flipped over his handlebars at a CG our group was at just before we arrived. No helmet, unconscious. Two paramedics in our group tended to him during the long wait for an ambulance. He nearly got to ride in a medical helicopter that day but he regained consciousness and was transported.
So cry liability, personal opinion, insurance or whatever. I like my brains and my kids brains the way they are. How do you like your brains? It only takes an instant to scramble them.
FWIW