Qole, I think part of the problem auto-hildonizing would be restricting all GTK programs to a small set of features. For example, Hildon applications can only have one menu, while GTK applications can have as many menus as they want. Hildon applications, while able to have more than one toolbar, would look pretty silly with 8 toolbars stacked along the bottom - not to mention there would be very little usable real estate.

That is not to say that some things could not be auto-hildonized, like hildon input method. I have no idea why all editable text areas do not have this automatically, other than the fact that code would have to be moved into GTK.

I am by no means an expert on this, but I can see why Hildon is a layer on top of GTK. Having small-screen support built into GTK would be another option, but would certainly be much more complicated and I think many applications (Xournal, Abiword, Gnumeric, etc) would not be quite as nice as their Hildonized counterparts.