Great. So if you are an experienced C programmer who can read and understand the source to The GIMP you can code a simple GNOME app.
You have a new programmer. They know some Python, Java or whatever. With the magic in Gtk+ and such every machine has bindings for those languages. Heck, RedHat and pretty much everyone and their dog does all the small fiddly bits in Python these days. So what do you point that new to GNOME (and probably Linux) programmer at to get them up to speed.
And if you can't see a tutorial on how to code a simple program as even possible, isn't that a huge problem?
Now you know why I still reach up to the shelf over my desk and grab _Practical Programming in Tcl and Tk_ anytime I need to bang out a quick graphical thing. Every time I consider jumping to GNOME with either Perl or trying some Python... but get daunted by the wall I'd have to climb over and just use what has documentation. Pretty sure I'm done before I'd even really get started on the Googlefoo to try a GNOME version. Tk is old and a bit klunky but a solid accurate manual beats the snot out of any theoretical tech advantage in my estimation.