I play live using only the onboard speakers at church every week. here are two songs i played live using just the onboard speakers both in performance and just jamming with live musicians including a bass player that had amplification.
http://uk.youtube.com/watch?v=aVU4lzgf5f4&feature=channel_page
http://uk.youtube.com/watch?v=ZJnGyIf0ozQ&feature=channel


Not all keyboards with onboard speakers can produce good sound coming from them. Fortunately the korg can.