aux is expressed in MBTI type code. the middle two letters are the dom/aux functions, I/E and J/P are just attitudes of those functions and also are traits on their own. also the MBTI letters are called preference pairs, as the N over S is just a preference, so is T over F and so on. i mean INTP has ESFJ inside, but its just not preferred.I wonder if the auxiliary function is actually as important for determining personality as the four letter/three letter(socionics) codes seem to indicate. As I understand it, Jung placed a lot of importance on the inferior function, and said very little about the auxiliary and secondary. The inferior function recognizes things that are unconscious. It describes what your struggle is, what your internal conflicts are. The inferior functions will "throw off" your primary. Inferior functions make people complex instead of just tidy archetypes. They seem to play a large role in the personality.
what jung did place quite bit of importance is the concept of differentiation and individuation(which goes quite much further than just typology). there is actually really much to be understood about aux and tert(also typology in general) and how they function from jungs work, its just that jung didnt directly write about them much, but they follow the same general concept of how the mind works in jungian perspective. or if you read what little jung said about aux and tert, then read what he wrote about other stuff than typology, you will be able to connect many things to aux/tert aswell.
i dont think that tert is easier to use than aux. the whole loop thing is an abnormal state where the opposite attitude of introversion/extraversion gets repressed(I types repress E obviously).As for the tertiary, if the "loop theory" is correct, than loops reflect the natural preferences and tendencies of a person. The third function is supposed to be easier to use then second function, if less likely to be used well, according to the "loop theory". Thus, it will be more common for an INTP to use Si, for instance, than to use Ne. Ne often requires conscious effort, while Si is more automatic. It seems odd, then, that the four letter/three letter codes include a description of a primary function and an auxiliary function, instead of a tertiary and inferior function.