CSDs are still optional in GTK3. True, they are forced on for the common dialogs, so if you use GTK's file chooser or font picker, then you're going to end up with them. But at least then they're child modal dialogs to a parent application.
The Gnome 3 desktop team decided to move every application possible to them, but there's no reason Xfce has to follow suit there.
Again, at least for now. It's certainly possible the GTK team will decide to enforce this and remove the option for window manager decorations in a future release.
I would really love to see a maintained fork of GTK2, however.
