Thank you for posting this article. While I am rather new to the study of personality theories, I've long been a fan of trying to figure them out. I agree that taking these tests without understanding the theories behind them can be a messy proposition. I've got a very weak/under-appreciated Fe. In college I tested as an INTP. The Dominant-Tertiary loop I was often in made me think I was much more of a practical, pure-logic thinker than I was so I ended up with a "T", and that same Ni/Ti loop over-analyzed all the questions regarding "lifestyle" so I ended up with a P.
Under the guidance of a proper counselor (and after many years of learning more about my true self) I have re-taken the test and am an INFJ. Studying the cognitive functions (CFs) of Ni/Fe/Ti/Se has really cracked open all the "whys" I didn't understand before.
Since I started learning about them, it frustrates me how often the CFs are overlooked. For instance, to me INFJ and INFP are like day and night. It helps that my best friend of many years is an INFP and it's like the OP says - superficially we seem similar but the way we think and use our CFs is very much opposite. I can't understand how anyone would confuse the two if they knew how CFs work. (No offense meant to any who
are confused; I didn't know the differences in the beginning either! But learn about the different CFs of each and you'll see what I mean).
It is very easy to get caught up in the basic 4-letters even knowing that CFs are there. I haven't pieced together the pattern to knowing which CFs go with which type but to illustrate how important they are, I had another of my best friends look up their type (she's a psychologist). She had me try to guess and I got her two middle letters totally inverted. She's an ENTJ - their CFs are Te Ni Se Fi. Seeing that, my guess of ESFJ is understandable when you see how much I over-think things and how dangerous I can be with just a little knowledge!.
Also, thank you for the reminder that information on cognition isn't completely cut-and-dry. That can be hard for me to wrap my brain around but I am finding Jungian theory and the MBTI (even with its weaknesses) to be too valuable to me as a learning tool to be discarded due to lack of black/white answers.