I use a mirror all the time when I ride. I ride a tandem (with my wife) and the mirror comes in handy if I want to change lanes because I’m not as young or flexible as I used to be. I still do head checks, the mirror just makes it so I don’t have to turn as far.
We get passed by closely all the time and the mirror has done nothing to prevent it. Even if I see someone coming close there is little I can do because I refuse to crash my bike (with my wife on it) to MAYBE avoid a incident with a cage.
I use a mirror as a convenience item not a safety item.