iTunes is more geared towards consumers. I can see ways to cram your collection in iTunes, but it won't (and never will) be pretty.
Don't professional tools offer management of recordings where you can use metadata like date, venue, session artists, booking agency, studio, gear lists, time breaks (for things like notes or references to other recordings) and such?
I'm not a musician, but I do see a whole lot of potential for such a tool. Something like Aperture for musicians.
Is there one solution for managing your music, photos, videos, documents and other data you download/generate? I don't think so. Files and folders are good for basic media management and easy to implement, but it just doesn't cut it when you need to manage >5000 songs or photos. That's when you need an application that's able to handle the specific properties of the kind of media you want to manage. Music has artist/album/genre/play count/rating, photos have rating/event/keywords/version, and so on.
The problem is two-fold. On the one side there's the problem of storing the data (which, like you said, should be the task of the OS), on the other hand there's the user interface for managing the data, and that's 100% the task of the application.
Amarok/Songbird/iTunes AREN'T music players! They are music management applications that also tend to play the songs because that's what you'll also want anyway.
If you want a music player, well... use a music player then!
/Funny F3 F3 F3 F3... (That's not UNIX, that's what I'm doing right now in Firefox
"Can you program?" "Well, I'm literate, if that's what you mean!"