The indicator doesn't display e.g. printers, print progress, several other apps and it was supposed to, worked like that before, without having the need to run both tray indicators (with notification area). One should suffice.
added Andrzej, because I can't test right now.
I think this may be because of Unity stuff being ported over to Gtk3. I've noticed many indicators or app-indicators that used to work in Ubuntu 12.04 no longer work in 12.10 because they only come in Gtk3 versions. Once Gtk3 versions of libxfce4-panel and plugin wrapper binary are available I will port indicator-plugin to it as well. The rest of the panel and all other plugins can probably still use Gtk2. Mark, how about merging the version we have now?
I'll try to merge tomorrow.