While I love the onboard speakers for the office/studio work, I dead plug them when I'm working. I want to hear what the audience hears--exactly what they hear. The keyboard speakers are great, but I discovered a few years ago that the audience is not hearing them--they're hearing what is coming from the PA system.
A classic example of Murphy's Law in this area was when a good friend and fellow entertainer was working a job at an Italian restaurant in Baltimore. He was playing and singing his heart out when all of the sudden, without warning, the PA, a Bose L1 Compact, went silent. Unfortunately, he didn't know it. He was monitoring himself through the keyboard speakers and never noticed the loss of sound.
I was sitting just a few feet from him when it happened, and let him know. He stopped playing, said he was taking a short break, and I fixed the problem. He had a bad 1/4-inch cable coming out of his mixer and going into the Bose. A spare cable quickly solved the problem and he was back from the break in less than 3 minutes.
Just food for thought,
Gary