Jo Shields posts about his intention to propose switching Rhythmbox for Banshee in Karmic. (I'm trying to convince him to apply to be a MOTU, or at least an Ubuntu member, so he can be on Planet Ubuntu.) He's not the first to suggest it, but he is in the right place to make the proposal for this release.
I'm personally quite happy with Rhythmbox, and haven't really tried Banshee, though for my use I imagine they are pretty similar. (Two things that would be definite benefits for me would be remembering what I was doing when I closed it when it starts, and understanding mix CDs better). I can certainly understand some of the arguments for switching as well, so I wouldn't be against it.
There is an idea on Brainstorm about this, and while Brainstorm can't capture the intricacies of the debate, it suggests that Rhythmbox is quite popular with the people that voted (though it's not clear how many had tried the alternatives.)
This post isn't really to argue one way or the other, or to attempt to cover all the criteria by which a decision will be made. My point is to emphasise that the Banshee developers have done themselves a great favour in this debate by making one aspect of switching easier. The have implemented importing from Rhythmbox. This means that any switch wouldn't mean that all users had to re-import their collection.
That's not everything that is involved in switching, and indeed, many of the issues around trying to change a default don't have good answers, and that's something we should work to improve as a community.
You don't have to spend time implementing importers for every similar application out there, but easing migration from commonly used apps can help users switch, and is a big benefit when trying to switch a large number of users painlessly. Also, while importing is useful, it's not the ideal solution. A common storage format, and shared storage would be superior in many ways for this purpose.