Hey, Buchan. I already put a few notes about the MDV process into the Fedora thread.
The main drawback to it is that it does come with a penalty of extra overhead on the infrastructure and development groups. It was a fairly obvious choice for Mandriva, which has a clear focus on accommodating as many regular end-users as possible, but it's not so obvious for Fedora, whose goals are not quite so evident. It's not clear that what Fedora actually wants to do is to make the lives of regular consumer users as easy as possible. If Fedora is focused more on aiding the development of Linux technologies, it may not make sense to spend resources on dual-stream updates. As the article says, it's a question that's not been settled yet.
BTW, KDE isn't in /backports because of the backports policy restriction which heavily discourages the updating of shared libraries in /backports, I believe.