I've been a fan of stereo since I was a kid, but the simple truth is ..... MOST areas of the room do NOT benifit from the stereo separation, and some areas can definatly suffer from the spread.
If the sound coming from your keyboard is still heard ABOVE the amplified signal, then chances are (thanx Johnny)the room will still hear your KB speakers too.
The idea that ONLY stereo is acceptable is rubbish. Phil Specter recorded hundreds of chart topping hits in the 60's in mono. Major studios ALWAYS check the mono mix before releasing a song and in most cases, a performance will reach more ears in a mono setting.

So, do I still prefer stereo? Sure ....... that's why I sit right in front of the keyboard speakers, and why I won't play a kb WITHOUT speakers. MY seat has to sound good to me. If that's good, then the rest of the night will be good. If I don't like the way it sounds ... then I'm crabby all night.

Don't fret about the minisqule loss of fidelity in a mono setting. 99% of the listening public is NOT in the right place to hear the true stereo spread anyway. It may have a "tiny" bit more presence and fullness but in a crowded room with chit chat and noise ........ a KB300 will pump out some major tone and fill most rooms. The only drawback is the horn .... it can tend to be rather harsh at high volumes, because it's a passive double piezo driver. At moderate levels .... that amp kicks. At loud levels, it STILL kicks .... it just hurts a little. (to those of us that still have good hearing, that is )

If your needs are not so demanding, and loud is not what you are after, then maybe a smaller, stereo set up would make you happier, but don't pass on the amp just because it's mono.