I imagine Apple would build iTunes library sharing right into OSX if they could, but god knows someone would have a problem with that. You can't even make a browser into a core OS component without someone whining about it.
iTunes 10 has this great button that looks like a dash, it's probably coloured orange, if you click it, the application more or less goes away. You'll find it in the upper left corner.
Anyway, I do agree that Apple should roll out a 'Home Sharing' preference pane, which is coupled with the components of iTunes that actually do library sharing.
I think the whole having iTunes open thing will annoy people who want to be able to "log out" and let someone else log into their account without the fustercluck of slowdown that OSX seems to incur with multiple simultaneous users. You log out, your iTunes closes, boom, no more home sharing. It definitely needs to be a system service/daemon.
OSX isn't rock stable, although it's saving grace is that it'll reboot pretty damned quickly and whatever your kids happen to have been watching on the Apple TV will resume from where it left off, even if you log out and hard reset (Steam.app crashed pretty spectacularly for me)