I can't think of any reason not to wear a helmet unless there is an extreme condition that would make wearing one while riding more dangerous than not wearing one. The situation that comes to mind is extreme temperatures/humidity in which wearing a helmet could contribute to heat stroke. I've come close to this riding in 100F+ heat in very humid south Louisiana and had to remove my helmet on at least one occasion to cool off. I replaced my helmet and continued after cooling down, but I'm wondering what presented the greatest probable danger under the circumstances: potential crash resulting in head injury or heat stroke?