Back when Napster was around I downloaded a fair amount of stuff, but even there jazz was virtually non-existent (especially the rare and/or oop stuff I was looking for. I've never been able to get Kaaza or any other peer-to-peer groups to even work - and there seems to be even less jazz on them.
I can say this: the stuff that I did find (and the music that I'm now downloading - legally - from emusic) has caused me to buy more CDs than ever before. The "problem" with being exposed to new artists is that, if you like what you hear, you want to buy more music by those artists.
I think that many of us who like and collect jazz have a hard time understanding the issues that the RIAA has with the general public regarding llegal downloading. Let's face it, the few dozen (even hundreds) who might d/l a Miles Davis album is a drop in the bucket to the hundreds of thousand who might d/l the latest Britney Spears or Radiohead CD.