I read through this thread and I read some of what others have posted, but I wanted to just jump in here with a more self-centered post - meaning it doesn't flow with the debate going on here and isn't directly related to Cognitive functions or personality theory, but I just wanted to throw my two cents in on the topic...
In response to the OP, any system will have its flaws and limitations - you work with and interact with enough man-made systems, I don't care how efficient, you will run into limitations. Cognitive functions are no exception.
As far as understanding others, in a personality/disorder sense, I don't care so I won't touch that.
For myself personally, personality theory is a hobby and is interesting, but it's not what matters to me, mostly because of that fact that I don't care about categorizing others. I'm a solid introvert and as such, I have no interest in other people. Any connection I have with others is through shared or connected past or current experiences. Admittedly though, I've had a lot of experiences that make relating to others relatively doable despite my personality.
As far as understanding my own psyche, there is only one division - that between ego and grace. These two terms I use based on my own understanding of concepts in Buddhist philosophy and Catholic mysticism, respectively.
Ego, I associate with: desire, attachment, ambition, hope, drive, passion, suffering, et al.
Grace, I associate with: stillness, letting go, discipline, acceptance, et al.
The interplay of these two forces is what interests me in myself and in turn, if I care to, I will apply this in understanding others, but only among the elements that are already familiar to me, if that makes any sense. The understanding of these concepts are not guided by training, schooling, system building, or any of the like. They are guided by an understanding found from trial-and-error, pain, and post-pain analysis. Suffering and emotion are the foundations of this "system." It's tough to explain but this is a very rough idea.
Any other type of system building, I find, is essentially a defensive act. Logical thinking, in my pov, is the act of separation that begins with the initial separation an individual has made within themselves, within the world they see, or between themselves and the world. At any rate, logical thinking, the foundation of system building, is at its heart an act of separation. And at the heart of separation is something that comes part and parcel with it - protection.
Not that protection is a bad thing. I guess what I'm saying is that if you really want to understand something/anything in the most thorough way, whether it's yourself or others, personality theory, economics, philosophy, et al. - in my experience, there is absolutely no substitute for the combination of trial-and-error, pain/failure, and analysis. The protection/logical system that comes almost effortlessly out of THAT cycle will be the most thorough and complete system you will have regarding any subject or topic.