What I mean is this:

If you get used to relying on boiler-plate code, you lose the ability ( thru lack of needing to do it), to do some low level customization which is needed to make some complicated apps run like you want. For most simple apps, its no problem, and Glade works great. But when you run into to one of the problems I'm talking about, you will find that the Glade boiler plate code gets in the way of a simple solution, and you spend more time trying to sort out the Glade, than if you just wrote everything from scratch.

Since it is not that hard to write the basic windows setup from scratch, why not just learn to do it that way to begin with?

To each their own. I mentioned Glade as a good solution for what it does, but I also mentioned that it is not a full blown IDE, and since half the work of building Gtk2 apps is setting up the event callbacks,( which you still need to do manually), I find it easier to build the windows from scratch.