The reason to use the mirror is to keep your situational awareness updated constantly, that way when a situation arrises you are already prepared and move yourself out of harms way before it's time to take the flyer into the ditch. Using a helmet mirror ( what I am acustomed too) is no different than the habbit you form when driving a car or motor cycle. Your eyes automaticly flick upward and take a brief glimps of the mirror, you are not focusing and concentrating on the happenings in the mirror, you are checking for things out of the ordinary, most times things will grab your attention if they warrant it. Same as driving a car. Most of us check our mirror unconciously, and dont even realize we are doing it until something in the mirror sets off the alarm bell.
Now the reason a mirror works for me is its faster for me to get a 6 check in the mirror than it is to try and turn my head and neck enough to get a good sight picture of what is behind me.I am 46 years old and flexable I am not, heck before I started using a mirror I was so concerned about holding my line thaton some of my stiffer days by the time I had turned and looked back and faced front again I couldnt remember what i had seen. I agree that if you are still flexable enough to turn and look it is probably better than my looking in a mirror, however I can take a look in my mirror just by flicking my eyes up and to the left. What's that take a 1/4 of a second. I still turn before any major manoovers. But in Michigan where I ride people in cars make great sport of trying to scare you out of your lycra by blasting the horn just as they pass you. or crowding you off the road or my personal favorite is the right hook while yelling "get on the sidewalk".
Are they a perfect solution, certainly not. But they do contribute to a larger overall picture of what is going on behind me.
The final thing that I would like to point out is periphrial vision. when you turn your head to look behind you, no matter how fast you do it, for just that long you are not facing the direction of travel. so if something does happen in front of you you will never see it. Now when I use my mirror to look behind me I still am facing forward and myperiphrial vision is still working for me. If something should happen in front of me while I am looking in the mirror, my periphrial vision should alert me to that. Same thing with the mirror, I have noticed many times that my periphrial vision alerts me to movement in the mirror and thus a quick check tells me if I need to focus on this object or not.