I think xmms is ugly, no matter what theme you put on it. RhythmBox is at least organized in a nice way.
Depends on how you see it; xmms is nice, small, and might not be pretty, but it works. RythmBox is big and slow to use with very large collections of files, but it is pretty.
It's a personal opinion, and there should be a choice for all. Sometimes there have to be compromises.
I for example would not be happy to loose Grip and xmms, but then again, I might be old fasion:-)
Regards/Casper