Not to convince you otherwise or anything, but the benefit of stereo isn't just what comes out of your stage monitors but what the audience hears. The stereo audio reduces phase cancellation that is inherent with all summed audio outputs, and it forces separate portions of your audio to reflect within the room from various angles towards the audience. I find that stereo adds a great amount clarity to the overall sound because of the phase reduction.

The only way to test and prove this to yourself is to run a stereo PA system in a room and try playing music from a CD player and/or your keyboard through it both in mono and stereo and see if it makes a difference to your ears when sitting where the audience sits.