I don't like wearing one, but do anyway (I pancaked from a good 6 feet in the air, bouncing on my belly and front of helmet once-almost bunny hopping a chain at full speed on asphalt ) I would have at least had a nasty gash maybe a concussion or worse without it. I also skidded and flipped down the asphalt at 50+ once without and ended up with a cracked skull and ridiculous road rash. (I actually was thrown from a rolling vehicle not a bike, but for comparison it is just like a high speed high side) so yeah.
wear a helmet, at least where speed is concerned, or sharing the road with cars.