Yes, I feel a mirror helps keep me safer. Properly set up, a mirror will give you more information than head turns alone because it only takes a split second to monitor so you can do it more often. I'm not sure I understand your "extra half second" comment. With my mirror I am always aware of what's going on behind me. If I feel the need to react in some way I would never wait until the last second. I would do something to get a motorists attention (and a reaction) long before I thought I had to bail. A head turn, (so they know I see them) issuing a slow signal and/or moving farther left in the lane early on has always gotten a favorable response for me. Although I like to think I'm ready to do so, I've never had to bail out yet.

I like the helmet mount because I can move my head slightly and see any area. I find the position of the mirror needs to be just right to get good field of view and avoid eye strain. Consequently I am very careful when setting down my helmet because if the mirror gets bumped out of position it takes several trys to get it re adjusted properly.

I think a mirror would only be a distraction to a newbie who had an extreme fear of traffic from the rear, who spent more time looking in it than looking in front. Riding without the immediately available information a mirror provides would be way more of a distraction for me.