I don't listed to music through iTunes at all. Only on my SqueezeBox and empeg. I use iTunes for everything you've mentioned.

With a good stand-alone tagger for Mac OS, I'd probably still continue to use iTunes for some of these purposes because it's a good managed interface.

Slimserver keeps a database of all my music but I have to wipe that on a regular basis for it to represent everything properly in its web interface and on my players. SlimServer reads the iTunes XML and finds out where the music is stored. It can also read iTunes playlists (including smart ones) and with a plugin, write back Played Count, Played Time and Skipped data. This data can be brought back into SlimServer after wiping its DB also by using a plugin. And another plugin lets you construct playlists based on all this data as well.