Setting APPMENU_DISPLAY_BOTH for GTK3 applications does not work in Ubuntu 12.04 LTS

To start the application with the menu enabled in the application and the panel, the following command has to be used:

APPMENU_DISPLAY_BOTH=1 appname

I recently discovered that this only works for non-gtk3 applications in Ubuntu 12.04 LTS, i.e. if you replace appname with applications such as gnome-terminal, gedit, evince, empathy, evolution, rhythmbox, nautilus, etc. only global menus will be displayed. However, if you start, for example, gimp or inkscape by using APPMENU_DISPLAY_BOTH, both global and local menus will be shown.

The questions is: why is APPMENU_DISPLAY_BOTH not taken into account when starting such gnome applications?

P.S. I know how to disable global menus in order to get the local ones (UBUNTU_MENUPROXY trick, removing appmenu-gtk/qt packages, removing the indicator-appmenu, etc.)

Thanks for the help!