Tony Mads is right. Stereo is for the benefit of the performer. And if I were still performing, I would certainly use it. When I last performed, I was using backing tracks mixed in stereo and I wanted to hear the sound the way I had created it. Same would be true using a keyboard, I would want to hear stereo. The audience doesn't know the difference. Now, there could be a few people here and there who are audiophiles and have trained their ear to a certain degree. But the average person in the audience can't tell much IMO. And some of the people I have seen playing music over the years and getting paid for it proves that to my satisfaction anyway.