This is an interesting discussion. Not being material scientists does make it difficult to separate the wheat from the chaff. It is my personal opinion that it is better to be safe than sorry. Helmets aren't that expensive...

I used to think that having one helmet was enough, but after thinking about it, I decided to have two helmets (one for road and one for mountain biking). I figure that, if nothing else, switching them out extends their lives and should something happen to one of them that I still have one.

What I've not been able to settle is the question of possible damage to the helmet when it falls from a height when it isn't on my head - like from the trunk to the ground. Does that cause damage to the helmet? I've been to different sites but there appears to be mixed opinion on this one. So I don't put it on my trunk or top of my car these days, just in case.