I use one. I don't like to ride without it, anywhere...I'm too jumpy so I like to know what is around me at all times.
I can only turn my head so far to either side, because of an injury, so that's the other reason I like having it.
I use a Cat-Eye Racing bar end mirror that works on drop bars & flat bars. It's cheap, easily adjusted, and allows me to see pretty far back and around.