As far as I can count, since the inception of Phonon, we had 2 ABI/API incompatible versions. of GStreamer. That's painfully often, but luckily we found a way to shield developers using KDE APIs from that.
Also note, that GStreamer is not the only backend we have for Phonon. VLC is enjoying a lot of work, and tends to give less headaches than GStreamer's backend. (It's not an option for everybody, since shipping VLC with its support for everything scares some distributors away for legal reasons, but it surely works very well.
Basically, the multimedia world is moving too fast (which in principle is good, since we need progress there) for providing stable APIs. That has to be done on top of that.
For KDE, Phonon prove to be very benefitial, since we didn't have to put all our eggs into one basket.