Unless you are very careful at making sure both systems (use mono out of the keyboard or sum to mono with a mixer) come out with the EXACT same volume, you can fool yourself into thinking that there is a difference between two things that, if heard under tighter controlled conditions, you might not hear a thing.

Primarily, only the Yamaha PSR series has had any serious accusations of a major difference between the mono out and summing the stereo outs externally. And, as of yet, to my knowledge, no-one has ever done a serious comparison of the two to find out definitively if this IS the case or not...

If I had a PSR, it would take me ten minutes to answer this question definitively. Sad that no Yamaha owners are willing to find out for sure...