It's not enough for me; I wear a helmet each and every time I ride (well, except on recent bike-shares in Paris, Stockholm, Vienna, and Munich, where none were available). But I've crashed bicycles a number of times in my life, even once crashing into a car as a teenager (flipped, bounced off the car, hit the ground) and still never hit my head.
Anyway, it's just interesting to read about what causes head injuries and what makes a cyclist prone (or not) to them.
Not anything that can be called rational evidence.
I've suffered several falls in which I heard a very loud crack: that of helmet hitting pavement, hard. That would have been my head, each time.