Human beings are not lizards. We have not evolved the ability to use our eyes independently to any great degree. It can be fatal to smack into something ahead of you because you are busy monitoring your mirror. It has happened. You cannot significantly change your odds of being rear ended by monitoring your six! You must learn to believe in the relative competence of other road users and focus your attention on what is ahead. Your ears (if they work) will tell you all you really need to know about what is going on behind you. When a mirror is priceless is when you need to change your position on the road and you need to know how that impacts following traffic. A mirror is a tool for planning offensive maneuvers. I really get tired of hearing how wonderful a tool it is for defensive protection. Five in one hundred drivers are deaf enough that a bicycle horn makes no impression on them. Bicyclists get overtaken much more than other road users. How many of you swear by your horns and its ability to force drivers to correct offensive maneuvers that put you in jeopardy... sigh... I see a lot of blame in this forum directed at 'those drivers'. Maybe, but I don't think we do all we can on our side to meet them half-way.
H