People are confusing the meaning of stereo and mono, when people talk stereo they are talking a stereo sound field (These days we have a multi-channel sound field) where the sound that the Crowd/Audience hears matches what they are seeing on stage, mono cannot create a sound field as it is a point source, (It can only come form one position) however if you have multiple inputs (A mono input from each performer) then you can mix these inputs to create the optimum sound field (Stereo or multi-channel depending on the number of output channels used) and quality of sound that matches what the Crowd/Audience sees.

Bill