nomorehelosforme, for the most part, it is as you say, down to personal preference, though some enlightened companies, even offshore operators, are making them mandatory for crews.
I liken it to seat belts, life jackets, immersion suits, life rafts, fire extinguishers, crash worthy seats etc. Never had to put any of them to the test, but they are sure nice to have to hand should your number come up.
A Bell 412 crew in Alaska were making a ferry flight at 10,000 feet when they encountered a severe vibration. So bad was it that their heads were smashed through the overhead windows and pounded against the door pillar, the overhead electrical panel fell open, instruments were unreadable, and to top it off both engines failed during the descent because they had been shaken out of their mounts. Had they not been wearing helmets I imagine their skulls would look similar to the gruesome photos posted earlier, certainly they would not have survived. And the cause? A pitch change link on one blade had let go.
And one who didn't survive. S-76 impacted the ground on its left side, trapping the co-pilot in his seat. The captain extricated himself immediately, and spent the next 45 minutes helping the crash crews get the co-pilot out, whereupon the captain collapsed and died. He had suffered a head injury that was not obvious to medical personel due to coming into contact with the broom closet at the time of impact.