The most important time for a mirror is when you encounter an unexpected gauntlet. Suppose as I approach a driver who's pulled over onto the narrow shoulder to talk on the phone, I see a car coming the other way; I gauge my own speed, I gauge the speed of the oncoming car, and I consider whether I'll pass the obstacle before or after the oncoming car. Now is not the time to look behind me; by this time I have to already know what's behind me. Maybe the road is all mine, and I can swerve safely. Or maybe it's a truck, and I'm going to have to hit the brakes and the ditch. Well, either way, I want to know my options as soon as possible.
I don't trust the mirror with that. If there's not enough time to do a check by turning my head, I don't trust just the mirror. Rather slow down, or move off to the side of the road and stop.
Mirror is good to avoid turning one's head multiple times. Once the situation is clear in the mirror, all it takes is one check (and then back to stage one in case there actually was a car in the mirror's blind spot).