My theory being that with pretty much any forward motion at the point of impact, you are going to get a fairly firm kiss from various parts of the cockpit, most likely from the circuit breaker panel behind as you head whips backwards and quite possibly the top of the instrument panel. In this instance a helmet will certainly help, and could very likely be the difference between remaining conscious or otherwise.
As Jose Jiminez once quipped, when Ed Sullivan asked if that was his crash helmet:
"I hope not!"
Pilots wear one anyway, in certain occupations.