See, that's exactly why I got a mirror, because looking over my shoulder long enough to what's going on behind ALWAYS makes me drift off my line. With a mirror, I can look at what's behind me while the stuff in front of me is still in my peripheral vision.

As for relying in my ears, my ears can't give the same acuity in terms of the approaching car's lane position, approach speed, and distance to me that a mirror can; not even close. And I simply am not going to trust drivers with my life when there's something I can employ to gain more situational awareness.

In the 5 years I've used a mirror, I've bailed out of the lane twice. Two times a car approached me so quickly that I either squeezed to the right against the curb, or jumped up a driveway apron onto the sidewalk. Those are two cases where the mirror definitely saved me from serious injury. In the first incident, the minivan did change lanes, but did so far too late to miss me if I'd have stayed in the left tire track. In the second one, the car passed me right after I got up on the sidewalk. The driver actually slowed down and apologized, admitting she didn't see me until I moved out of her way. (She was almost as mortified as I was angry; I hope she pays more attention now.)

I don't see a mirror as a panacea, but I certainly wouldn't operate in traffic without it. Anything that increases your situational awareness and gives you more information to make sound decisions in traffic can't be bad.