Despite Sebastien Bacher's (software engineer at Canonical) initial proposal to stay on GTK 3.8 for Ubuntu 14.04 LTS, GTK 3.10 landed in the Ubuntu 14.04 Trusty Tahr proposed repositories.
According to an Ubuntu Desktop mailing list message, the rationale behind updating GTK to version 3.10 in Ubuntu 14.04 is to make "app developers happier (and we expect some of them to want to stay on the LTS for some time), and unblock some work for Ubuntu GNOME" and also, this should "bring performance improvement to our Unity menus".
In the same message, Sebastien Bacher explains that Canonical plans to fork gnome-settings-daemon and gnome-control-center for Unity 7 (see: Unity Control Center @ Launchpad) and stay on the current version for Trusty - if that happens, it would benefit Ubuntu GNOME which could ship with the latest GSD/GCC.
Furthermore, for Ubuntu 14.04 Trusty Tahr, Canonical wants to add menubars back to GNOME applications that no longer use a traditional style menubar in recent versions, such as Nautilus, Evince, GNOME Calculator and so on: "the plan is to upstream those patches, and to make the menubar be displayed in a conditional way depending of the environment, that's what is done already in Gedit". The developers are also looking for a way to get client side decorations apps to look more consistent, like adding the window manager decorations back and hiding the close button for example.
That got me thinking that we could get GNOME 3.10 apps in Ubuntu 14.04 but in a recent message, Sebastien Bacher explains that only 3.10 applications that bring mostly bug fixes and no UI redesign can land in Trusty:
"Yes, we really want the UI consistency/working menus for the LTS. I'm not sure about 3.10, I've looked at those and we did a few selected updates for components which bring mostly bug fixes and no UI redesign. I doubt we are going to take apps using GtkHeaderBar this cycle though, those need more integration work".