Originally Posted by John Forester
I got along for years without using a mirror. I use one now because my neck joints have become stiff and painful, so that I can get a few looks behind before I really need to turn my head. I still think that the probability is extremely small that one will be able to distinguish the motorist who will hit you from behind from the motorist who will steer past.
John, it has nothing to do with determining "the motorist who will hit you from behind from the motorist who will steer past," and everything to do with finding a gap in fast moving traffic.
I find a mirror quite handy when I am merging with 45MPH traffic, and need to find a gap in that traffic while I am also keeping an eye on the road ahead. When I spot a suitable gap, I can then turn my head to confirm the gap... and offer a subtle signal that I am moving to the left. If I can, I will then also offer a proper hand signal. Then I will make my move. In dense, fast traffic, the mirror makes all the difference in the world.
Watching for the motorist "that may hit you" is impossible.