Jim, I did for two decades and never had a complaint. In fact, I dead head plugged off the onboard speakers and just used the Bose L1 for a monitor because I was hearing what the audience heard. And, there was a single instance that those onboard speakers got me in trouble. On that particular day, I forgot to turn on the PA and didn't realize it until someone said they couldn't hear me hardly at all. I was hearing everything just fine through the onboard keyboard speakers, but my Bose L1 PAS ans sub were not turned on. Damned that was embarrassing.
Keep in mind that I used my special piano voice when I played piano, so mono sampling was never an issue. Same was true with some of the sax and guitar voices that I massaged for mono usage. No one in the audience ever knew.
Now, Don Mason and DNJ will tell you that you MUST have stereo and your audiences will hear that stereo output, but that has never been my experience in all these years of working with various sound systems, both conventional and vertical array.
Personally, I prefer vertical array systems over any conventional sound system for many reasons, the most of which is sound dispersion. When I was still performing, which was until August of last year, my Bose L1 Compact covered a 210-degree spread of any room that I played, and the falloff from keyboard to the back of most venues was less than 10-percent. You cannot achieve this with most other sound systems, and never with any conventional system - stereo or mono!
The second best feature of vertical array systems was virtually no feedback at all, which was never the case with conventional systems, even after I switched to feedback rejecting mics and my Crown CM-311A, which has incredible feedback rejection. I don't know exactly how this is accomplished, but it sure made me a happy camper.
Some folks are old school, I guess, when it comes to sound technology, but I can assure you that during the past two decades, the vertical array sound systems have made great strides in providing huge improvements in overall sound quality and clarity. Keep in mind that Vertical Array sound systems have been around in churches and theaters since the mid 1960s, but back then the columns were huge, most armed with 6 to 10 8-inch Jensen speakers in a housing that weighed up to 200 pounds and was made of 1-inch thick particle board and covered with black velvet fabric. The subs were 25-inch monsters that were placed beneath the stages or pulpits and had 6-inch maximum throws. You could blow out a candle with a sub using very low frequencies at a distance of 10 feet. They were brutes!
Today's vertical array systems utilize tiny speakers made with synthetic cones and small permanent magnets, often facing in different directions to facilitate better spacial coverage. Now that I have retired, I sold my second Bose L1 Compact to a local OMB entertainer that plays an old Korg, has a female singer, and a guitar/sax/fiddle player partner that works with him on his nite club job. When he goes out alone on the senior circuit, it's just him, the Korg and the Bose and a headset mic.
Hope his helps,
Gary
You always have a backup in place, and you get to hear that wonderful stereo! Frank has great prices!