Here's my theory about INXPs. So.., get ready to flame ~
Now, from my observations on this forum, there are many INTPs are INFPs who seem to question their type. I don't have anything empirical, but I'd say that the two are possibly the most 'confused' types out of the entire lot. The following is an explanation as to why that may be.
The idea is that INFPs and INTPs can't survive in the real world without being a bit more like their counterpart. And thus they have adapted to be more like one another.
The major weakness of an INFP is the fact that they are insanely idealistic. I speak from experience that many of my ideas are very unpratical/unpragmatic. They JUST don't work. This is because whilst they may seem fitting to the INFP, they lack a sort of logical coherence. Furthermore, some of them are even based on assumptions, with factors not being taken into account and some random contradictions not being considered. HOWEVER, the good thing is that INFPs are often quite reflective. They see their weaknesses, and they do something about it. In this case, they develop a more analytical mind, using logic more, becoming more rational. I think most of us are familiar with Darwin's theory of evolution - it's something like that. The way they adapt - learn from other types, read more,
The same could be said for INTPs. An INTP might need to develop their 'F' side to make themselves more likable to others. It might be unfair of me to say this, but whilst INFPs might lack practicality with ideas, INTPs sometimes present themselves to be mechanical. Their overwhelming logical nature would make them incompatible with other types, who do not have the energy and effort to analyze situations like an INTP does. Thus, to get along, an INTP might hang around feelers more in order to develop their relationships with others. (The same might be said of INTJs, but they're quite stubborn for change.) Afterall, humans can't survive alone, they need to cooperate with others as well.
Now you might say that the two cases I have taken are simply the extremes. All I would say is that there needs to be some sort of equilibrium in INXPs. If they weren't at the opposite of extremes, then there would be no need to change anyways, because they're already at a sort of equilibrium. Thus, because they are constantly needing to be at equilibrium, that is why they are probably the most 'confused' type out of the entire lot.
And of course, at the end of every INFP idea: hope nobody takes any offence.