It may be easier for compatibility with GTK 3.0, when it comes out (which isn't for a while). We haven't decided how we'll handle deprecated GTK functions generated by using an old Glade.
Maybe you should try to set to branches of Geany and support one for GTK 2.6 and another one for 3.0. This would satisfy both users with older GTK and the ones, that would like to get all this new functionality.