A couple of further thoughts about keeping a track of the segments used and the sequence that they were used in:

1) I don't see why it couldn't work for the midi styles instruments as well as the realtrack ones
2) (assuming that each 'generation' required a relatively small data file to record this information) it could allow you to step back through prior versions either one instrument at a time or for the whole song. This would be a cool feature. At the moment when I get something sounding 'pretty good' I tend to stop experimenting in case I lose it. If I had the freedom to say "actually the one two generations ago was the best" and to go back to it then that would allow me to play with the song more freely and (hopefully) lead to better results. This kind of multi-level undo would probably only work within a single session and a single song but it would be really useful.

If I hadn't seen such riches I could live with being poor.