I have changed my position on using mirrors whilst cycling.
My position has been for years that unless you were astride a recumbent, or had impaired neck movement, mirrors were a silly distraction to the task at hand.
The reason for this is that the perils that face a cyclist are far and away predominantly ahead of him. We face hazards in the form of road surface impairments, obstructions and debris, and the turning movements of other traffic.
My feeling has been that until one is preparing for a movement that will potentially cross the path of following traffic, knowing what traffic behind you is doing is of no concern.
This not always true.
I now recommend the use of mirrors for cyclists
When one first takes to the roadway with a defensive position that uses body language to communicate his presence, he ought to use a mirror.
I can describe how traffic reacts to the presence of a slow moving vehicle, but few who are new to driving their bicycles in the traffic lane will really be confident of that until they observe it for themselves.
So when you grow weary of the close passes and the near misses of suddenly opened doors, right-hooks and left crosses and begin to assert your right to the roadway as an equal user of the public roadway, use a mirror.
Count the times you nearly get "runned over". (You might be surprised to see that it never happens!)
Observe how overtaking traffic yields to your presence. Take note of the delays you create. You will soon learn that any such expectation is simply a myth.
When you get tired of watching for such things that never happen, and have proved to yourself how even high speed traffic have no difficulties in overtaking you, you can put the mirrors away.
Because then you will have seen for yourself that the perils you face are in front of you, not behind you.