It's all optional software.
And Songbird looks like a ripoff of iTunes.
Songbird is much more capable than iTunes. You can sync to any device (including iPods, with the exception of the iPod Touch), it uses less resources than iTunes, and doesn't come with Quicktime (which I hate to no end). Songbird also has add-ons, which, last time I recall, iTunes did not. Yeah, the UI looks like iTunes, but it was made to, and you can customize the UI using add-ons.
Also, if it's optional software, why did iTunes, after completely uninstalling Quicktime from my friends computer, say that Quicktime was needed, and reinstall it? Sure doesn't seem optional to me.