[Commercial Developer Hat - speaking on behalf of myself and not the company]My day job is maintaining an application called VMware Workstation. The application is among one of the oldest and largest Gtk+ codebases at 15 years old, and has survived almost every major GNOME regime change from Gtk+ 1.3 to Bonobo to the D-Bus revolution to GIO to Gtk+ 3. As you might imagine, we're stuck using a few deprecated APIs (like, e.g., stock icons, of which we get thousands of warnings for alone while compiling).
As such, we hit all kinds of these random issues through the ages, where subtle changes are made in Gtk+ and we spend our time tracking them down through Google and grepping hopefully through git logs. It's tiresome and it is rapidly turning my beard gray.
The runtime warning would be incredibly helpful for tracking down these kinds of things in development - we build shipped versions of our application with these warnings suppressed anyways, and the GNOME team has already done(/undone ;) this in many other occasions:
https://git.gnome.org/browse/glib/commit/?id=85e9455f68c6cc6093a1ede1c5f15587bba08b51
https://git.gnome.org/browse/glib/commit/?id=f2f66bfe45f41d18e7437341e5ebf4c70d815958
-Andrew Walton