I've always thought that was just a line from the manufacturers to get you to buy a new helmet more often.

F'instance, I've got a foam helmet that I've crashed twice, both times my head the dirt pretty hard (hard enough to really hurt and possibly concuss, had I been bare-headed) but apart from the slight dent and scraping on the exterior, no damage to the helmet. The foam didn't noticeably crush and after checking it thoroughly in good light, I can't see any cracks or anything. I don't see why this helmet won't protect me just as well the next time I go down.