See, here's my thoughts.
In terms of crash protection I'm not at all confident it will save my life in a huge impact. I might get lucky, but if tank commander Sven Volvo smashes into me at 40 MPH I'm likely doomed.
My thoughts (and they are just thoughts as I'm not in the habit of testing it for myself) is the kind of lower speed impact where it might result in an unpleasant but probably not life-threatening injury is where the lid might help. But even then its not guaranteed, and there's also the small possibility of torque injury to the neck cause by enlarging the circumference of the head.
So I wear my lid in the reckoning (not belief) that it won't save me from anything really nasty, it might reduce the likelyhood or severity of a lesser injury. But it might not. Conversely, not wearing it won't protect me from anything at all.
So I wear it and hope for the best really, while expecting that the worst is still probably coming my way if the brown stuff hits the bladed rotating object.
It's on my noggin because "might" is better odds than "wont", but I ain't kidding myself.