It's worth pointing out that styles ARE midi files, and since they are repeated so often in a song they tend to sound more like mechanized accompaniment than SMF's which can be given a lot more variety. I randomize the velocity of my SMF drums and backing instruments so they have a human feel that doesn't tire the ears. You can do that with styles too but since each phrase is just a few bars long and then repeated infinitely the randomization is short lived and becomes it's own predictable pattern. I use styles now and then but SMF's just give me a better and more complex sound. And it makes a difference that I am a pianist - styles really tie down your left hand and I prefer the flow of having both hands working together melodically as opposed to splitting up their duties. With SMF's and MP3's you also have the option of performing with an acoustic piano, thus widening the range of gigs you can do.