Mirrors are a great tool for physical therapists, personal trainers, powerlifters, or even the average weekend warrior. They can be very helpful when used to maintain or correct proper form and positioning during an exercise.
However, there are also times when mirrors can have unfavorable effects. Check out this video of a single leg deadlift being performed in front of a mirror.
Yes, coaching and cueing can definitely improve poorly executed exercises, but as human beings it is natural to want to look in the mirror. We like to be able to see what our bodies are doing. The best cues in the world may not be enough to combat this urge of the visual system in some individuals.
Now take a look at this video with the same individual performing a single leg deadlift without a mirror in front of him.
Take the mirror away and the exercise becomes great again. The individual maintains a neutral spine throughout the whole movement, allowing strain on the back to be reduced. Removing the mirror also promotes the individual to reinforce his kinesthetic awareness, or his recognition of where his body is located in space as he moves. This is an extremely important skill to refine because we don't walk around with a mirror in front of us all day!
So whether you're a clinician, trainer, aspiring student, or just working out on your own, it is important to understand that a mirror can act either as an affordance or a constraint. Use it to your advantage when needed, but do not become reliant on the mirror in your exercise programs.