Comment 1 for bug 1359249

Yup, that is the case. So far, only Xubuntu and Ubuntu Studio are taken into account.
http://bazaar.launchpad.net/~indicator-applet-developers/indicator-sound/trunk.14.04/view/head:/src/service.vala#L209

I guess the reason the Xfce session isn't there is because nobody complained about it before.