I once read of a comment by a helmet manufacturer, along the lines of " If a helmet can be damaged by a small drop, like off the tank of a bike, then i wouldnt wear it in the first place."
In modern helmets, a lot of the impact is absorbed by the liner ( believe me, I know). The outer shell provides abrasion resistance, protects the liner from damage holds the whole package to gether, and most importantly gives you somewhere to mount cameras and intercom units.
I replace my helmet when the chin strap get abraded, when the helmet gets loose on the head, or impacts something solid with my head in it.