Wait - I'm dumb as shit here - I just thought of the only skydiving helmet that actually is a protective device IMHO - the ProTec. Ask yourself - why don't people wear protecs? They are bigger, bulkier, and just flat out not cool looking. But fact of the matter is, if you make a mistake and your head hits the ground, wouldn't you rather have the protec and its shock absorbing/dissipating charecteristics?***
Uhmmm... my boyfriend made a low turn under a stiletto 120 and he was wearing a protec (he usually wears a full face helmet, but that was a "fun jump"). When he impacted the ground his chin strap broke (the thing that didn't resist was the plastic peace you use to close the strap, i don't know how is it called in english) and his helmet flew away... He broke his femour (some pieces...), the great part of the bones in his face and had a head trauma (fortunately with no consequences).
It's difficult to say if the protec was a positive or negative factor: probably it absorbed part of the impact anyway before breaking, but wearing a full face helmet would have prevented all the hurts in his face...
Anyway... no helmet at all and he would be dead... but I don't think protecs are better than other helmets...