I read somewhere recently that injuries due to head strikes on the road comprise a rather small percentage of all motorcycle crash-related injuries. That, as I recall, included both those who did and did not wear helmets. Apparently most of the injuries were to the body and extrematies, not the head.
While anytime the helmet hits the ground in a get-off and should be replaced, it is, according to those conducting accident studies (NHTSA, I believe, as well as the insurance industry) the least likely of injuries for some reason. I wish I could cite the magazine article I read. It was interesting. This is not to justify riding helmetless, however, as other studies have shown that one is 60 times more likely to need the services of a safety helmet when riding a motorcycle in town than out on the highway. I choose to wear mine all the time but that is my choice. YMMV
"Don't sweat the small stuff---it's all small stuff"
'09 650 VStrom