The study accomplished by John Forester found that highly experienced club cyclist still crashed once or twice a year. So minor crashes are BY FAR the most common by probably 10,000 to 1.
It is highly unusual for a rider to hit his head in these crashes the human reaction system being designed to prevent just that thing. But since most people don't fall down much their reactions to protect their heads are dulled by non-use and helmets are probably a good idea because of this.
I just want to make the point that you wear a helmet to protect you from relatively minor accidents such as scrapped heads (hey, us bald guys have to worry about our looks don't we - scabs and scars are ugly) up to perhaps concussions. Not to say that a concussion isn't a pretty serious injury but it can't nearly compete with death.
The ONLY way to prevent serious to fatal injuries is to prevent collisions involving motor vehicles and a helmets will have no pertinent effect in that sort of case. The very best tool in that case is the thinking brain of the cycle operator.