The military has requisitioned many studies on the benefits of helmets and I believe that the civilian world should take note.
I certainly believe that ALL EMS operators should wear them as a matter of course due to the unknown missions that are asked of the crew.
The number of US EMS operators not wearing helmets surprises me no end. Get your crews into them. How many head injuries do you guys respond to each year on the road? Doesn't that hit home at all? Throw a comb in your pocket if you are worried about "helmet hair".
UK and Australian aircraft flying EMS/SAR all wear helmets. Seems to be the norm Down Under. Can't comment on other parts of the globe.