There's another point to bracing, and it's a bit squeamish: People have survived incidents but been unable to release safety belts because their arms were both broken. Bracing can reduce the risk of this.
Airbags are a nice idea (fnarr) but equipment levels reflect cost, weight, practicality and - the big one - the probability of need. The chances of a passenger jet making a forced landing on water are miniscule, so it's equipped with basic life jackets or just floatation cushions. (And in many areas, the cold will kill you before the water). Conversely, the risk exposure's higher in a North Sea helicopter so the norm is immersion suits, insulating layers, enclosive lifejackets (
MMEmatty's design
) air rebreathers and sometimes 4-point restraints.
If I felt the need, I wonder if I'd be allowed to to travel in a crash helmet...
