Crab;
Back in the 90's there was a move to make NS pilots wear helmets, a study was run and the findings were that as NS pilots flew far more hours annually than their on-shore or military counterparts (at that time pilots with my employer were running at a rolling 790+hours on their 365 day totals) that there was greater risk to health caused by the increased weight on the head being affected by vibration and therefore causing spinal damage high up in the neck.
There was also mumbling about how the pax would feel seeing the pilots with an apparently enhanced form of protection denied to the customers. Personally I would not have wanted to wear a helmet for those lengths of time, sometimes the grow bag/lifejacket combination was bloody uncomfortable enough for 8 airborne hours in warm weather.
SND