Why use Songbird over iTunes? The add-ons. I'm already enjoying the musical experience more with Songbird than I ever did with iTunes. While I'm listening to a song, Songbird displays metadata about the song's artist from Last.fm, shows me if the artist is on tour and coming to my city, and scrapes the net for relevant reviews, photos, and videos. It can also suggest me artists similar to the ones I like. It slices, it dices, and it's dishwasher-safe.
The first add-on I downloaded, The Exorcist, scanned my library for ghosts (songs in Songbird's library XML that are missing the mp3) and displayed any possibly song duplicates. This made it very easy to do a little maintenance on my music library.
Some add-ons I mean to try in the very near future are Seeqpod support (searches the web for an mp3 and let's you download to your library) and a display for lyrics of your current song (*bumps Rock Band mic to Expert*).
The only feature of iTunes I miss is how iTunes would copy files into your library and create artist/album folders on-the-fly. How about it, Pioneers of the Inevitable?