Just a quick note on this subject. As an official "middle-aged old guy" (and an Fi-dominant INFP), I'm going to side with proteanmix in favor of the importance of Fe and the slipperiness and subjectivity of Fi.
When I was young, I would have said the opposite. The rules of society seemed fickle and subjective. They changed from town to town and time to time. If you traveled from a small town to a big city, the rules of social intercourse were turned on their head. Social rules in leisure were different from social rules in the workplace. Social rules in the civilian world were different from social rules in the military. Thus, I would have said that Fe people (as the interpreters of social rules) worshipped a fiction, a mere shell. By contrast, my Fi seemed truth-seeking, betterment-oriented, geared toward finding universal truths, etc.
Now that I'm a bit older and wiser, I've pretty much reversed my opinion. For example, I've learned that social rules are Darwinian and have usually developed a certain way for important, objective reasons. (The rules for small towns are different from the rules for cities because the small town environment is different from the city environment; the rules reflect the environment.) By contrast, a youthful Fi is comprised of whatever bullshit, feelgood fictions and fairy-tales the Fi person imbibed in childhood. The ultimate test of Fi will ultimately be how well Fi functions in the real world. If social rules are Darwinian and an Fi person has no respect for social rules, than a young Fi person is probably going to become a candidate for a Darwin Award when he dies in some spectacularly stupid way.
Also, Fi is not nearly so solid as it may seem to outsiders. It's always going through slow transformation due to the betterment-seeking aspect. A rebuff or disappointment can cause it to spasm and whiplash in an opposite direction. If social rules are Darwinian and an Fi person has no respect for social rules, then rebuffs and disappointments are going to be frequent. I have seen and studied quite a few older INFPs. And from what I see, Fi can end up in quite a few wildly different places at the end. And some of them aren't very pretty at all.
Frankly, seeing what Fi can become at the end, I don't have much respect for the betterment-seeking and truthseeking qualities of Fi at all. I like Fe much more. Fe is connected to something objective (something outside the individual) and thus leads more reliably and consistently to a reasonable measure of wisdom, happiness, and ability to fit in with the community around oneself.
I could go on, but I'll stop there. Let me just say this. I see Dante's "Divine Comedy" as analogy of how Fe and Fi interact. That is, Dante (the pilgrim, the seeker of truth, the talented but untutored poet) is Fi. Hell (the land to be traveled and interpreted) is life and society. Virgil (the guide, the mentor, the interpreter, the accomplished poet, the "Voice of Reason") is Fe.
FREE MonkeyNotes Study Guide Summary-The Divine Comedy:The Inferno by Dante Alighieri(Dante's Inferno)-CHARACTER ANALYSIS/DANTE THE PILGRIM/VIRGIL-Free Book Notes Online Study Guide Book Report Plot Synopsis Analysis
Fi has raw talent but doesn't know how to express itself and is fundamentally disconnected from the society around it. Fe people, as social adepts and interpreters, have a valuable function to play in terms of accommodating the individual and society to each other. The only variable is the value we place on society. If one considers social rules as important, then Fe people deserve great respect for their talents and their willingness to serve as guides and interpreters for those who are less adept.
In my old age, I would argue that social rules are very important, especially to an Fi:
1) for Darwinian reasons, at a minimum;
2) as the medium and the means through which an Fi accomplishes his dream of helping others (i.e., if you wish to save a drowning person, it's probably good to know how to swim); and
3) as the source of Fi values; initially childhood Fi values came from society (fairy tales, parents, siblings, etc.; they don't just appear out of a vacuum, after all), and in order to achieve real maturity the Fi person will at some point have to re-link them back to society (IMO). If he can't do so, frustration, rejection, bitterness, and loneliness tend to be the result. It may make for good art, but it's not a pretty sight in real life. (Edit: Fortunately, most INFPs manage to pull their heads out of their asses at some point in life and recognize the benefits of conforming to social expectations to some extent.)
Just my opinion and my two cents.
(Just a drive-by post; I have a busy week ahead and won't be able to respond further. )