I find a mirror a large advantage on poor-quality roads. It enables me to keep an eye on that pothole or crack or pile of glass I'm trying to avoid and at the same time make sure a car isn't going to cream me if I swerve out to avoid it. Although I can certainly turn around and look back, I'd prefer not to take my eyes off the road that long if the conditions are dangerous.

Different people like different kinds of mirrors (helmet, glasses, bar, fork, etc.) and that's okay. We don't really need to agree on that.

The arguments against a mirror are not compelling, in my opinion. Many say they don't need it. Many fear being made fun of by their peers (it is considered very dorky).