Hello!
Even being 2+ years old I don't think the BeOS interface is dated at all. Just because it isn't covered with candy looking GUI elements like OSX and WinXP are doesn't mean that it is dated. The other OSes have gone overboard a bit I think (though at first I liked them.) This is especially true considering how much all this eye candy slows them both down.
But now, the other OSes look better. There is a candy-like fashion, and apparently everybody likes it. If even windows switched from the cold war design to something smoother, there must be a good reason.
I guess (I can be wrong on this one, but...) one single window manager is enough, but changing the look and feel is important. The main reason is not that you can change the look and feel. The main reason is that it allows an evolution of the default look and feel.
Well, the first target is to recreate BeOS, so it's clear what the first look and feel will be. But after that? If the candy-like fashion goes on, we need a default candy-like look & feel. Otherwise. we should be able to develop whatever the new fashion is with a minimum of programming effort.