I have held for a long time that there is something to be gained by studying functions and their activity in the human psyche in pairs. Personally I do not believe that any one function manifests in a person, either by way of type or individual instance, without being paired with another. The pairing is what brings the functions into focus, allows them to see one side of things, to make information "tangible" to the brain instead of being just raw perception, which is impossible to process; combination refines perception and makes it useful to us.
I believe there is a great deal of insight into the types to be gained by studying this with its relevance to Model A, and I think a lot of how different types utilize and experience the functions can be explained by looking at how functional pairing relates to the Socionic psyche.
I'll throw one example out there that strrrng and I discussed.
Let's look at usage of Si in IEI and EIE. In both types, the most easily accessible form of Si is Si blocked with Fe; Fe is a strong, native function for both types, meaning that anything paired with it is going to be consciously accessible with some ease, whether it is preferable or not.
Now, looking at both of these type's natural functional makeup, we can glean two things immediatly.
For IEIs, Ni being their base, any use of Si is going to be somewhat "undermining" to their natural state; Ni and Si are both IP functions, and since Ni is IEI's dominant function, it is going to be a bother or a nuissance, most likely, to pay any attention toSi, because it takes them directly out of what they most want to focus on. However, they have moderately strong Si, it being their role function and thus the same temperament as their base, so while it may be a bother, it's not something they usually have a hard time with; more they would just rather not pay attention to it in a very general sense. Role functions serve mostly as a "tool," something we can make use of when we feel like we must, but we are not exceptionally confident in, and prefer to avoid using whenever possible.
For EIEs, with Fe base, use of Si is, while unnatural and not preferred, also not much trouble at all, because it does not undermine their natural base; indeed, with practice, it can be highly commensurate. Being Ni creative, they are accustomed to bending their second function to their will and using it with impunity, and thus they will likely try to do the same with whatever "takes its place" in the pairing shift to Si: use it in service of Fe. However, given that Si is their weakest function, they are likely to be either reckless or clumsy in its usage, to over- or under-do just about everything related to it; they will likely try hard, but not yield exceptional results, and thus become extremely frustrated. In this way, PoLRs are often probably our greatest source of frustration: we can see, on a preconscious level, that we have potential interest in it, that it "should" be something we exercise control over because of the natural pairing with our dominant function and similar use to our creative function, but we are thwarted at every turn, and not likely to develop or maintain any serious investment in making use of it in our daily lives.