I wrote a big thing about what Orobas was asking regarding how we make a cohesive system out of all the bits that we put together and then lost it! For me, I see it like a Christmas tree which I am adding decorations to. I may buy a box of decorations, but only some fit my needs for that particular tree. Even if the ornaments in the package may all be of a similar stye, perhaps the colour, shape or size of some fit better than others to make a cohesive whole. There is nothing wrong with tinsel or angels, or bells or lights or whatever kinds of ornaments there are. Not one in and of itself though is going to be sufficient to provide the variety and balance required for an aesthetically pleasing tree (which is one of the main purposes of the tree). When I decorate, all the ornaments and trimmings need to fit together in a balanced manner and they need to be adaptable to the needs of that particular tree (certain branches being sparse, assymetricallity, what kind of top there is for the angel to go on etc). The tree is what is most important, not the ornaments (elements of a system).
Therefore, I see some great things about MBTI, function theory, enneagram, but I also believe that all have limitations because each person of a certain category may exhibit some unique characteristics depending on their life experiences, interactions with other people and desire to accommodate. For example, because EJCC is an ESTJ who grew up with NT and NF parents, she has a different way of interacting with those types than an ESTJ who may have grown up only around SJs. An ENTP that decides there is value in having harmonious relationships with the people at work may make it a priority to understand how to work with NFPs instead of purposefully poking their buttons for their own amusement. Depending on the kind of earlier experiences someone has, they may be more closed or open, more secure or insecure. This of course will affect all of their interactions. Therefore, I see the systems as having useful elements for giving a shorthand to discuss differences as well as pointing me in the right general direction for deciding how to interact with a certain type, but I am very aware that it is only a general guide, not a definitive one.
Maybe it's a bit like a car that comes out of the factory as a standard model, but which can have a variety of different extra elements added. The basic framework is the same, but it may be a luxury model, an economy model, be more aesthetically pleasing, have a custom paintjob or tires, be more sporty, have certain amenities etc. It's still a car and understanding how that make and model of a car works is useful, but the extras are also important to differentiate between as well.
To continue the tree comparison, I believe this is why I am so reluctant to change my internal structure easily when handed new pieces of information that require a complete reworking. I'm willing if I'm convinced of the need, but it's like suggesting an entirely different colour scheme or style of decorating and everything that I've worked so hard on needs to be changed and new items procured which fit equally well. I want to be really convinced that that kind of work is actually necessary before starting in.