(And remember
GNOME & KDE are multi-platform - so I don't
think GNOME & KDE apps can use ALSA directly.)
They don't have to use ALSA directly, but they can use a
common library which uses ALSA directly like polypaudio.
That's true. KDE also runs on other operating systems like BSD
or even Win, so they still need an abstraction like
polypaudio, gstreamer or portmidi, and this one needs to be
prepared for all other supported platforms.
Best regards
ce