At the typography BoF at GUADEC you asked why the GNOME font settings where
routed through XSETTINGS and not by writing to an XML fontconfig config file.
I said there's probably no reason for that and that's just the way Owen or
whoever did the work did it.
But then I thought about it more and I'm confident that it's because up until
about 2007 / 2008 we didn't have file monitoring in glib, and as such Gtk+
couldn't pickup fontconfig changes (including config changes) on the fly. So
apps had to be restarted for font settings to take effect, which is a
non-starter. Thought, it's still possible to write a config file, send a
XSETTINGS signal, then on the client side reload fontconfig config, which is
actually how we do it now, but gnome-settings-daemon detects config changes,
so there's no need for a tweak tool to send any signal, it can just write a
config file and it will be picked up in a second or two...