Actually, this is not true. Find me a program that can extract the scores from a complex piece of music and attribute the right notes to the right instrument/voice. I haven't found one yet. But maybe that's because I am not versed in the music analysis software available. Never needed it. Now I think of it, it would be really handy for my job though. If you know of one, please let me know. Especially one that can analyse the music live and then transposes it to midi notes/OSC without a hefty lag.

Well, composers, conductors and improvisers (e.g. jazz, folk) need to have it, so that would put it to several hundred thousands, not "less than a thousand".