tcs , I have no dog in this fight, either. I've never used a helmet-mounted light. However, I do wonder about this: when I've seen such lights, they are mounted directly on the top of the helmet (for obvious reasons). I have crashed and wrecked a few helmets over the years, but never put damage in that spot; rather, I usually land on the side or very rear of the helmet. So, I wonder if a helmet light is theoretically dangerous, but practically not quite as dangerous...?

Also, if a helmet-mounted light does reduce a helmet's effectiveness, that must be weighed against the reduced likelihood of a crash from using the light.