I mix in some of the better SMF files when singing, and playing over them. One reason I bought the Roland G70, was for it's excellent handling of midi.

Before that, I used to use my lap top with an Echo sound card, and Notation Composer(MidiNotate). I would arrange the file as a lead sheet with chords, mute the parts I played, and read right off the laptop.

Both ways have advantages and drawbacks. I think a mixture adds interest.

Bernie