According to the spec at https://blueprints.launchpad.net/ubuntu/+spec/desktop-karmic-default-media-player-choice it appears as if Ubuntu is trying to use banshee as the default media player for Ubuntu.
I like banshee myself and I have replaced rhythmbox with banshee on all of my installs. I don't think this makes my opinions biased in this sense. I love nice GUI's and one of the bigger reasons I chose banshee over rhythmbox is the GUI.
Back to the spec. Ubuntu appears to be choosing banshee due to the size it takes up on the CD compared to rythmbox. If I have to put my two cents in here, I would say this is also a plus because rhythmbox support has stopped, and banshee support seems to be great.
Until Next Time