I'm not sure how much I can really add here, but I played with this thing a little about ten years ago, and it was fairly popular at the time.
The general idea was that you specified a chord progression (or outlined a song, with head, chorus, various repeats, etc.), then you picked a style (various flavors of jazz, for instance), maybe a tempo variation (ballad vs. something more upbeat), and a selection of instruments, and it would then generate parts for the various instruments.
You certainly weren't going to record these and sell them, but it did a surprisingly good job, of playing whatever progression you came up with, in varying styles. You could literally just choose an alternative style from a menu (perhaps even for a single instrument, like swapping out your bass player or drummer, I forget), and hear the same thing played in a completely different way.
The MIDI stuff is just a distraction here. It helps if you have the capability, but as gare said, that's not what this thing is about.
As I recall, I read something (on a usenet group back in the days...) about an add-on set with some Coltrane stuff, but they were going through a phase where they were no longer supporting Macs (though maybe they've come back?) and you couldn't get some of this stuff anymore.
I'm not making a recommendation one way or the other, though personally I'd rather spend my time playing an instrument than a computer these days. But it was surprisingly good software back then, and I can imagine it being a useful educational/practice tool, depending on what you're looking for.
