Well, one thing that could be done is to try and create a decent style starting with a good midifile. Some companies use professional or semi-professional musician to record midifiles of very good quality and a software like Styleworks pro has an option to create a style starting from a midifile. You just have to choose what part of the midifile will be your intro, your basic or advanced section, your ending, etc. I guess that before doing this you will have to transpose all the tracks to the same tonality, though.
True, this way you will never match the complexity of styles (like the ones from Roland) that have different rhytm tracks -even in the same section- for major, minor or seventh chords, but at least you will be able to play your favorite song without being tied to a midifile; in other words you will be able to lenghten or shorten your song at will.