I'll chime in with what is just a personal belief and as such has no affect on this debate.

While sparring without headgear I find my defense to be a lot sharper. I don not feel that this is a result of the headgear increasing my confidence, I feel it is subconcious....Without the headgear, I feel more aware. I have better vision and a better sense of my opponents resence. With the headgear I look one way; straight forward. Without my eyes are allowed to dart and look for tells. I'll check his hands, his shoulder, even his knee to see if he intends to step in. I feel headgear limits all this.

However, I feel that we are missing one great benefit of headgear (I mentioned it earlier and Thomas acknowledged.) Last Monday while sparring I was caught wioth an extremely hard looping right hand. However it landed on the lower part of the rear of my head. I frooze. I don't think it was intentional but three more straight rights to my face were landed before I had the sense to say "hold on a second I'm stunned." I was wearing headgear and I dread to think the result had I not been. It pained me for quite a while and I was grateful then for headgear.