The problems I have often seen are: 1) confusion as to where on the music score you need to continue after looking down at the keyboard and 2) in an orchestra people who look at the score and at the keyboard have no time left to look at the conductor.
Of course when you start learning you need to visualize the keyboard in your head, just like you visualize the lefthand side as well. The mirror approach is a nice trick. But in the end it should not be necessary to ever look at the keyboard while playing. After years of playing the same instrument the keys will not have moved. You should develop some muscle memory as to which moves brings you to which key. And marked keys may also help.
My wife sometimes looks at her keyboard and more often than not it causes here to lose track of where in the music piece she is. It's hard enough to keep track of where you are when you need to press a chin switch and have varifocal lenses. (While you continue to look at the score it suddenly becomes all fuzzy for a moment.)
Of course when you start learning you need to visualize the keyboard in your head, just like you visualize the lefthand side as well. The mirror approach is a nice trick. But in the end it should not be necessary to ever look at the keyboard while playing. After years of playing the same instrument the keys will not have moved. You should develop some muscle memory as to which moves brings you to which key. And marked keys may also help.
My wife sometimes looks at her keyboard and more often than not it causes here to lose track of where in the music piece she is. It's hard enough to keep track of where you are when you need to press a chin switch and have varifocal lenses. (While you continue to look at the score it suddenly becomes all fuzzy for a moment.)