I wonder what you do with the information you get from your mirror. The risk of distraction is minor when obtaining that information, but processing and deciding on action must be a significant distraction. What do you do with that information? It's true there are cars behind us all the time, and you don't need a mirror to know that. The most important information I would get from a mirror is if a car is going to hit me and what evasive action I should take. That's a big decision and it consumes time (distraction). I think it would make me paranoid and give me "false positives".