In the US, at least, helmets seem to be the exception, not the rule. I know of very few in use offshore, those being flights for the MMS, gov't inspectors, sometimes. There is a place for them, as John Eacott noted, on fires, longline, etc, but for most routine transport I don't think they're necessary. I wore a helmet for years in the military, & haven't worn one regularly since. I tried one on a MMS flight once, & removed it very quickly. Admittedly it was a poor fit, but the noise reduction was much less than my custom headset, & I couldn't understand radio traffic at all, because the audio quality was so poor. I've been flying since 1968, & fortunately I've never been in a situation where the helmet would have been any help.
I do realize this is a subjective area, & I won't argue with anyone who wants to wear a helmet, as long as he won't argue with me for not wearing one.