-- Last Updated: Oct-23-12 1:23 PM EST --
Bike helmets are very light and are intended to work just "one time". That would be true of any helmet with a really severe impact, but unlike paddling and climbing helmets, bike helmets are NOT made to tolerate numerous minor impacts.
I'll qualify that. I have an old Bell bike helmet from the early 80s, and it weighs ten times what a modern bike helmet weighs (that's why I replaced it ages ago, with another Bell). That monster could probably take quite a few big hits.