Jose,

Your comparison would be correct if the piano was the only sound emanating from your keyboard, but for most of us arranger keyboard players the whole point is to simulate playing as a band. We do need separation for realistic sound. Unless you envision all the band members piled up one on top of another, a single speaker will not cut it. I want to have separation between individual instrument sounds - I think we all do. Our outputs HAVE to be stereo, unless we have instruments with individual outputs for each solo and accompaniment track, and have a dozen PASes connected in (though I am sure Bose would like us to do that).

In addition, a piano is a "stereo" instrument by its nature, especially when you are playing it - the base is emanating from the left, and the treble tones/harmonics from the right. This is part and parcel of a realistic piano reproduction.

Regards,
Alex