I don't wear one but advocate personal preference here.
For military ops the helment forms part of the uniform and is worn irrespective.
For civilian twin-ops corporate, offshore, RPT etc work I see little need for a helment. Not that it's a consideration for me as the pilot, but if the passengers see the pilots wearing helmets...let's just say it doesn't auger well and the management probably has an opinion about that.
For S/E ops, particularly any external load or mountainous/jungle terrain work, then wearing of the helmet is a wise option. There have been many times on S/E jobs that I've thought I should be wearing a helmet but haven't, but not so many when flying a twin.
Fortunately, in all instances I've never had to call on the benefit of wearing one.