I will say this, and it's pure conjecture on my part, but here goes...

I do believe that the peripheal senses (not just vision, but spatial awareness, and proximity sensitivity,etc)
are limited and distorted by helmet use. The deprivation caused can play with the gyronomic equilibrium of the head/body relationship. This is the key to balance, which in turn is the basic dynamic principle of riding a bicycle.

Ironically, The only severe accidents I've ever had, I happened to be wearing a helmet.

I do believe that the times I've saved myself in hairy near-misses, I was saved by the uncanny principle of spatial awareness, and my body's ability to self-correct the gyroscopic aligment by "steering" with my head. [/theory]

However, the nasty crash that I mentioned earlier, where I scraped my head on the pavement at 20+mph, was caused by a thrown chain on a ghetto-rig hub. That was unavoidable, and the helmet prevented me from severe lacerations on the face and head, if not from a concussion.