not everyone can use their aux function. universal traits doesent exist(even tho some people think they do and attempt to type based on those traits, which can lead to mistyping), because typology isnt about traits, its about different types of cognition(cognitive functions and their hierarchy in the psyche).
what comes to not using Fe, cuz its immature. undeveloped functions are about you not being consciously aware of them, not being able to consciously direct/use them, contents of the functions mixing up with the other contents of the unconscious which leads to different sorts of cognitive dissonances(you hate and love someone at the same time would be an example of this happening with undeveloped F) or just simply not recognizing the value of the function, thus ignoring what it 'says'.
even if some of the four functions were not developed, that doesent mean that they are not there, you do judge value of things, give definitions to things, perceive what is and possibilities, whether you do those things consciously or not. the thing is that if you dont do those things consciously, its not you who is being in control, but you are being controlled by the functions which gets mixed up with your unconscious emotional complexes around things.
i agree with cafe that INFJs might come off as rude/impolite/dick without realizing it, even if they would just like to help(i think they share that with ITPs). i dont think this not realizing how one comes off to others is about Fe, Fe is about making judgments of value, based on objective criteria, which
can be social niceties, but definitely is not limited to that, but it can just as well be something else. for example one INFJ(female) has almost made my INFP(male) friend cry few times for calling him
http://en.wikipedia.org/wiki/Chronic_obstructive_pulmonary_disease and his name(lol that disease has long/hard name in english) and criticizing him from some related stuff, and she just wanted to help.. and she isnt someone who lacks social awareness in general, unlike the INFP(who btw would never say such things to someone and would cry himself if he made someone else cry or hurt someones feelings).
if you want some indication how to differentiate INFP from INFJ, INFPs try to have some structure inside their heads(which can be shaken, but also balanced by Fi) and their external world has next to no structure, except when Te comes to play at times. INFJs on the other hand try to have structure in their external world, while going all over the place inside, except when Ti manages to make some sense of it all.