Lets see now, the last time I looked at that Roland, the speakers were touching each other, so there really is no stereo separation there.

When you plug into the mono plug of all of the keyboards I've owned, the left and right channels are automatically connected together, thereby summing the sounds to a single output, which also combines the effects.

So the only beneficiary of stereo would be the player if he or she is monitoring themselves using the onboard speaker system of the keyboard. Or, an audience if there were two speakers spaced wide apart.
Think we've had this discussion several times and no one every seems to agree.

Gary