Grew up in a very well entrenched MC family. Didn't wear a helmet on bicycles. Smacked the back of my head multiple times on a bicycle, where you smell funny things, etc.
Smacked the back of my head many times on MC's wearing a helmet where I didn't notice a thing.
One of the findings in section 12.1 (pages 416-419) of the Hurt Report.
47. The use of the safety helmet is the single critical factor in the prevention or reduction of head injury; the safety helmet which complies with FMVSS 218 is a significantly effective injury countermeasure.
If you have a $5 head wear a $5 head cover.
Long and short, the younger you are the more impact you can take in repetition. The older you are the more impact you take in, repetition compounds.
I think that some these views are slowly starting to change as long term effects of repeated injuries (even without visible symtoms) are starting to be monitored. Think Football, Hockey issues of late.