Hi Miami Mo
Its just like the first stereo, a live group (Lets just say a Jazz group) are spread out across the stage, not on top of each other, and stereo keyboards try to give the same effect, (Your backing group and playing is spread out between the 2 speakers) if however you are just playing piano, then as it only comes from one position, mono is fine.
In truth, if you are playing for dancing, then probably no one would notice whether it was mono or stereo, (The punters are 2 busy dancing) but in a concert situation the difference stands out.
In the final analysis, it will be the decision of the player as to what best suits the venue. (If it helps, if you look at most electronic keyboard/organ concerts, they always use stereo)
Interesting observation

Bill