Arch Linux

Actually since libpulse is already a dep of gst-plugins-good, phonon-gstreamer should just replace libpulse with gst-plugins-good in its dep list

Can you be more specific about what the problem is? What exactly breaks if gst-plugins-good is not installed? The wiki link you posted has nothing to do with phonon

Without gst-plugins-good installed, (phonon-)gstreamer will not use pulse at all even if there's a running instance. So depending on libpulse alone is not enough (unless you only want it as an makedep).

I see no reason it should be an optdep instead of a dep. I think it's a proper and common practise that packages which support pulse will have the "client-side" stuff ready (usually libpulse, but in this case gst-plugins-good as well), while leaving the "server-side" optional. So if users prefer plain ALSA, they simply do not install the package "pulseaudio" or disable autospawn.

If it's made an optdep so that users can pull less packages, then libpulse should be removed from dep list of "phonon-gstreamer" and be only makedep of "phonon", because it alone won't make phonon-gstreamer work with pulse anyway.

EDIT: I just tested with amarok playback (hence phonon-qt4, but I assume it will be the same for -qt5) myself and confirm that libpulse is not required as an makedep for phonon-gstreamer. phonon seems to need it as makedep though (or maybe just certain deps of libpulse).

PS: In case I didn't elaborate well in text, I do it again with graph, so basically the output chain is like:
phonon -> phonon-gstreamer -> gst+plugins-base+base-libs -> plugins-good -> libpulse -> pulseaudio

It's alright to do this:
phonon -> phonon-gstreamer -> gst+plugins-base+base-libs (-> plugins-good -> libpulse -> pulseaudio)
which means libpulse will be only makedep of phonon and nothing of phonon-gstreamer

or IMHO preferably this:
phonon -> phonon-gstreamer -> gst+plugins-base+base-libs -> plugins-good -> libpulse (-> pulseaudio)

but as you can see, this is just silly:
phonon -> phonon-gstreamer -> gst+plugins-base+base-libs (-> plugins-good) -> libpulse (-> pulseaudio)

You're right that it doesn't make sense for phonon-gstreamer to depend on libpulse, it's not linked to it and phonon already depends on it. I'll remove the dependency.