@Martin: Maybe I misunderstood your remark, but that's the way xdg base dirs work... The first entry in the variable has the highest priority, so XDG_DATA_DIRS="/etc/xdg/xdg-xubuntu:/usr/local/share:/usr/share" will give /etc/xdg/xdg-xubuntu >> /usr/local/share >> /usr/share. So the default value is really /etc/xdg/xdg-xubuntu.
@Martin: Maybe I misunderstood your remark, but that's the way xdg base dirs work... The first entry in the variable has the highest priority, so XDG_DATA_ DIRS="/ etc/xdg/ xdg-xubuntu: /usr/local/ share:/ usr/share" will give /etc/xdg/ xdg-xubuntu >> /usr/local/share >> /usr/share. So the default value is really /etc/xdg/ xdg-xubuntu.