IMHO, the aircraft helmets I have seen (and occasionally used) are next to useless. They are typically a very thin shell, with almost no internal padding, and in the event that you actually hit something hard, I son't believe that they would do anything to reduce the overall impact that your skull experiences. The best you could hope for is a slight load spreading effect, which may or may not matter, depending on the size of the impact area - if it's a small, localised impact area, then chances are the object will penetrate the helmet anyway.
If you compare a/c helmets with motorcycle or car racing helmets, it's night & day. The latter really do offer substantial protection.
But wearing one of these precludes the use of headphones.....
I also believe that situational awareness is a more difficult art in an a/c than on the ground in a car or on a bike - you don't have nice neat lane markings on an essentially 2D surface, and other a/c can come at you at wildly varying speeds, at any angle - the speed differentials are potentially far greater, it's all in 3D, and you can't hear them coming! And they may not have seen you either.... Personally, wearing a helmet makes it more difficult to maintain awareness.
Proper belts are THE WAY to go - from what I have read, most head injuries in a/c are caused by heads hitting panels due to lack of upper body restraint. "Lap belts only" are the culprit.
Proper belts obviates the need for helmets in 99.34% of cases. (OK, I made that up, but you get the drift).
In the 0.66% of cases where that doesn't work - rather than helmets, I reckon airbags would work a treat in a/c - don't know how much these things weigh, but it certainly seems to be a future way forward....
A