I love my handlebar mounted mirror. I find it helpful in traffic. I also ride solo on rural roads without shoulders, and like to make sure that I know what's behind me and whether or not it is planning to give me space. Also, I went to too many rock shows in my youth and my hearing isn't the best, especially with wind.
I call it my "nerd mirror" when I ride with roadies. In smaller groups people appreciate it because I'm able to call car back without the need for anyone to turn their heads.