I always wear a helmet whether i am riding the road bike, mountain bike or motorcycle. In NYS, it's the law to wear a helmet while operating a motorcycle, but optional on a bicycle.
To each their own but in my family, we wear helmets, for our safety and protection. Since they are light and very ventilated, there is no real reason not to because accidents happen all the time. Last Fall, a rider in our group crashed due to (of all things) railroad tracks. That led to three other riders going down with him. One rider was knocked out but thanks to his helmet, nothing more than that happened. Of course, he did not finish the ride and went to the ER to get himself checked out, but in the end all was good.
Another crash last Fall was due to a stone on the road. He went down hard and cracked his helmet, broke his collarbone but stayed conscious. Wrecked the bike pretty good too but the bike was not wearing a helmet, lol.