We see them on most crews who fly in EMS, law enforcement, firefighting and military rotorcraft. But helmets are the exception rather than the rule in ENG flying. They are hot, expensive, un-cool...and they can ruin a reporter's hairstyle.
But helmets also unquestionably save lives.
The pilot who was wearing this helmet during an accident had to buy a new one, but he lived to tell friends about surviving the crash. Had there been no helmet, he says it's questionable whether he would be talking at all (click image to enlarge).
NEHA, the FAA and NTSB recommend that all ENG crews consider equipping themselves with helmets. Consider equipping not only pilots, but also photographers and reporters. In ENG flying, we often fly similar profiles to our emergency services colleagues...low and slow, with sometimes few places to stuff the ship when she quits.
Helmets have prevented both fatalities and serious brain inujuries during rotorcraft accidents.
Helmets can also provide protection for the crew during bird strikes. 
Take a look at these results of a recent collision between a helicopter and a flock of geese. (click on image to see more)
Both front windshields were taken out and the pilot was not wearing a helmet. The pilot suffered major lacerations and disorientation. No one else on board was injured.