Due to the switch to pulseaudio a few releases back, the situation with xfce4-mixer has become more and more troublesome. The multimedia-buttons (especially unmute) don't work well anymore – a problem actually addressed in xfce4-volumed by prioritizing pulseaudio over alsa – et al.
All in all xfce4-mixer was designed for alsa and that's what it works best with. Since Xubuntu uses pulseaudio instead and indicator-sound in the panel it makes far more sense to ship a mixer that can bring forward the advantages of pulseaudio (e.g. per application volume-levels), i.e. pavucontrol.
This is why I propose to replace xfce4-mixer with pavucontrol, it doesn't make sense to ship two mixers, this would only cause a confusing situation (=mix up ;) ) for users.
Obviously this replacement will have consequences, e.g. favoring pavucontrol in the "sound settings" of indicator-sound. (bug #951745)
If Xubuntu wants this, the it should go ahead. Approved, but do it quickly.