I like your description of the tertiary and 4th functions as teenager and toddler respectively ;) It makes a lot of sense though, especially when considered in the context of their environment. I have started listening to some of the personality hacker podcast (specifically on the loops right now, because that is really speaking to me), and there is just so much to this theory. So many levels.

We used MBTI and the HBDI (http://www.hbdi.com/HBDI-book/c/) models when I was in the public service... mostly they were used as a way of learning more about yourself and the people you work with so that you can get along better, but I understand that sometimes they can be used to screen people in or out of certain professions, which is just all kinds of wrong.