Just to add to what Frank said, you're not limited any longer because, for example, you like the synth sounds on your korg, but don't like the piano.

OK, so you can add the piano plug in board to the Korg, but what if you don't like that very much either ? Maybe you're Yamaha doesn't have the best synth sounds on it ? Add the AN 150, which sounds pretty good, but it's only 5 notes of poly.. not very good for playing sustained pads. You're Roland has electric pianos that don't please you ? Ok, then you have to live with whatever Roland offers for expansion boards.

The point is, you're stuck in the hiearchy that the manufacturers give you to work with. Not so with soft synths and wavetables. Maybe an outstanding guitar sound does cost you $200. Still might be better than getting a synth or even an expansion board.

OK, you can add samples to your existing board. Trouble is, at least on the Motif, it's a very involved process to get from point a to point b. It isn't as simple as that. There are a lot of side roads that have to be taken from raw sample to ( good sounding ) finished patch. It's much easier to do in a software module.

AJ