That's half of my complaint, that the GTK+ devs consider "stable" to mean "frozen". You are supposed to code against the latest completed version (e.g. the final GTK+ 3), which will not receive compatible feature updates, and then if you want to get the new features you have to port your code to the next version.

Whereas under the old model (GTK+ 2.x) it was forwards compatible (if that is the right word) - you code against a stable but actively developed version, and your code will work with no or minimal changes for multiple years, and you will receive the benefits of new versions (historical examples: changes to file open/save dialogs, theming changes, window resize grippers, dragging windows from menubars, ...).