I did have the same thought myself because as storage space and bandwidth get cheaper and bigger mp3 has one advantage - the sound is less dependant on the listeners equipment. If the listener does not have your keyboard, or if the listener just has a very basic gm sound chip, your performance is either unplayable or out of your control in terms of the gm sounds. Whereas with mp3, even within its limits of sound compression, your performance is preserved much better than a midi file.