[Nocap]

You could say that each function is a theory in itself. Jung designed a system that would cover every way we can take in information, and every way we can deal with it. Mathematically he covered every base. The only one I'll say that doesn't NECESSARILY apply for is Thinking and Feeling. I imagine there's another way to sort information than just personal decisions vs. logical decisions.

As far as intuition and sensing goes... information is either concrete or abstract. There's not really any middle ground. Same with introverted and extraverted.

The problems don't come from improper or inappropriate application, but in having too narrow an idea of the functions' capabilities, and how they interact with one another. Both of these two problems are usually rooted in assuming that the superficial RESULTS of the functions, are the functions themselves, rather than figuring out what the mind is 'doing' and understanding that what we see is only the manifestation of what's happening in the brain. It's likely that no two people express their function combinations the same way.

On top of all that I'd wager at least 50% of anything any person does can be traced back to more than just one function. It's highly complicated, but all the bases are covered.

And yes, the OP was talking about MBTI specifically, but a lot of people don't really know the difference between the two, and I'll take any venue to explain their differences.