The idea that GA flying is safer than driving simply doesn't cut it. It probably isn't even safer than peacetime military flying - particularly if you're flying something 'characterful' rather than a C172.
On the surface I have to agree with this. But, there's more to it that that. I've been flying a year longer than I've been driving. I suppose I've driven more than flown, but not by much. I've never had an injury in a car, nor plane. But I've had four cars destroyed in collisions, while I drove, but never lost a plane. I've had two friends killed while flying, but they were both doing very silly low level maneuvering, which did not work - entirely flyable aircraft were crashed fatally.
If I could not have a shoulder harness (hopefully four point), I'd wear a helmet. I always wear reduced flammability clothing, and suitable footwear, in case on fire (never sandals). I always wear a life jacket when flying on/off the water.
I have found a sense of security when flying helicopters in wearing a helmet, and I'm sure that in some aircraft types (ejection seats) a helmet is an outstanding idea. For regular GA aircraft, I don't perceive a risk which a helmet would reduce.