Xfce 4.12 Planned For March, GTK3 Still Uncertain
Phoronix: Xfce 4.12 Planned For March, GTK3 Still Uncertain
While most of the Linux desktop talk these days is about Unity, KDE, and GNOME, advancing in a quiet but steadfast manner is Xfce. The next major release of Xfce, version 4.12, is due out in March. It's undecided at this time though whether Xfce 4.12 will use GTK2 or GTK3...
What is needed to be done to get it ported to gtk3
The reason they don't want gtk3 is that it takes significantly more memory and is noticeable slower, which makes it not "fun" to port it. The developers are volunteers, using their spare time for hacking so the "fun" part is their only salary.
If you seriously want gtk3 support for xfce, then you should build the currently ported xfce libs (they are in xfce git), and show that the memory usage have improved in latest gtk3 release or git - or optimize memory usage in gtk3.
You should also give the xfce devs some "proof" that gtk3 will no longer break theming every release in future.
Those two issues fixed in gtk3 and I am sure xfce-4.12 will get support for gtk3.