Ade blogged recently that some of the people sharing beers around a table one evening were ponder the question "Why isn't Phonon just a thin wrapper around GStreamer?" and couldn't come up with a good answer.
The answer is pretty simple, however: Phonon is a thin layer over GStreamer when it is used with the GStreamer back-end.
What it isn't is a thin layer for only GStreamer, and the reason for that is similarly straightforward: our target platforms are your computers, where "your" is defined as "people of the world". Reality is that not all of your computers use GStreamer, and it isn't KDE's role to try and ignore that reality.
In that sense it's just like how KDE runs also on BSD or OpenSolaris: it isn't our job to make people running those OSes to switch to Linux.
In that sense it's just like how Qt is a "thin layer" over X11, but not exclusively as it supports at least four other windowing systems and a multitude of operating systems.
We wrote the GStreamer backend, bringing thousands of applications to GStreamer (which begs one to ask why people are looking gift horses in the mouth), because we believe in GStreamer as a solution. If in five year's time GStreamer has taken over the world (as defined by "our users") then everyone wins and nobody loses; if that doesn't happen KDE still doesn't lose.
So why is Phonon not a thin layer over only GStreamer? Because we cater to the whole world, today and tomorrow, even as we take care of our preferred choices and platforms through our efforts.