Phonon is only an abstraction layer with API and ABI guarantees so every time a new technology comes and goes, all app developers wouldn't have to port their existing codebases.
Funny story:
During KDE 4.0 development, KDE introduced Solid library. Which abstracted HAL.
HAL was Linux'es "Hardware Abstraction Layer".
So HAL developers mocked KDE and got some tshirts that said "KDE Abstracted my abstraction layer".
1-2 years later HAL was deprecated. And Solid got a new upower/udev backend and no KDE developer had to port away their code from HAL to anything else.
Phonon's situation is similar. Xine, gstream, vlc, these technologies come and go. KDE apps don't care.
Also Phonon can get a QuickTime backend when it's built for Mac or something else for Windows.
Funny story:
During KDE 4.0 development, KDE introduced Solid library. Which abstracted HAL.
HAL was Linux'es "Hardware Abstraction Layer".
So HAL developers mocked KDE and got some tshirts that said "KDE Abstracted my abstraction layer".
1-2 years later HAL was deprecated. And Solid got a new upower/udev backend and no KDE developer had to port away their code from HAL to anything else.
Phonon's situation is similar. Xine, gstream, vlc, these technologies come and go. KDE apps don't care.
Also Phonon can get a QuickTime backend when it's built for Mac or something else for Windows.