This is a difficult subject. I learned the following on a recent cross country tour with lots of traffic:
Most bikers had no mirror.
Some had a helmet mounted mirror.
I was one of the few who had a drop bar mounted large mirror.
My experience was mixed. This mirror needed often fine tuning to be useful. You also have to actually look at it which means you are not looking at the wheel 6 inches in front of you as in a pace line.
The mirror is useful if you ride alone and keep an eye out for things coming up from behind.