I believe mirrors have a place in a Dojo, when checking the severity of injuries.
As for using mirrors when training, I believe they detract a practitioner from "entering" themselves (complete focus) while training.
As I enter my 38th year as a practitioner, I am seeing more and more the wisdom of the old Masters.
Mirrors can become a "crutch" to young (in time of training) students. They may tend to focus more on what they look like rather than what the technique(s) "feels" like.
Bottom line is any training is better than no training.
If your Dojo believes in wall to wall mirrors and the instruction is legitimate, go with it. The training is what matters most.
My experience has shown that most traditionally based Dojo set up the training area in a traditional way, no mirrors.
Students should base training decisions on the legitimacy of the Ryu, not mirrors.
We have mirrors I personally don't look in them. They can be usefull for adults but in the kids classes I really wish we could cover them and the younger the kids the worst it is. The only thing I find them usefull for in the kids classes is it's like having eyes in the back of your head after you catch them fooling around a couple times when your not even looking in their direction they learn to behave all the time.