Unless I've misunderstood information I've read, the noise attenuation of the average helmet exceeds that of most headsets. I have a Gentex SPH5, much lighter than my SPH3 but I've not been wearing it for the last few years since I've been flying offshore in a multi-pilot crew.
When I did wear it, I had customers balk at my helmet, while they were offered plain old David Clarks. My explanation that satisfied most complainers was that I sling loads - shortline, longline and aerial application, somewhat more hazardous operations and I'm just used to wearing it all the time rather than switching back and forth or carrying an extra headset for when I don't feel I need it. Most people accepted that arguement.
Biggest problem I ever had was getting my head in and out of the door frame of a 500 while longlining. I could never do it without banging my head pretty well every time and I'm just under 6 foot tall! But I still wore it and felt funny without it.
I don't think there is any question that helmets save lives and I will be happy to accept any mandatory ruling. For the passengers however, you'll need to find some kind of disposable headliner if it ever comes to that.