Mike, it is simply because keyboards in a live band situation require external amplification and onboard speakers would not be enough. Because onboard speakers make people think of home keyboards it is thought that leaving them off makes a keyboard look more professional. While the first reason I gave is valid the second isn't. Personally I like to be able to play a keyboard without having to set up an amp at home so I think having onboard speakers is a plus. I used a Yamaha PSR-550 with a band at a number of gigs running it into an amp but at home I just use it with its onboard speaker system.

Onboard speakers are very cool !!!!