Keirsey & Jung are using the same word in VERY different ways. They use such different ways of categorizing personality that I'd hardly combine the two... One is behavioral, the other psychological in the most non-tangible way.
All types can be rational & use rational thinking, of course. J-dom approach the world as something to be organized into something that makes sense (Thinking classifies what something is & Feeling assigns value). P-dom, however, are "aware" of things - they "see" what is & what is not. They don't need to use extensive lines of reasoning & classifications to "see" things - those things just
are. Jung says it's difficult for a Ji-dom to grasp how the Pi-dom experiences their perceptions as objective (aka they don't relate it to themselves), that they see it as just an idea that may be true, likely, or possible. Whether it's true, likely, or possible is determined with Je, so of course there is still use of rational reasoning. The Ji-dom, however, is highly aware of their own subjectivity in the sense that they KNOW their judgments/classifications are created by themselves, and their aim is to hammer out their reasoning into something airtight so it can be accepted by others. This is very easy to see in INPs especially...it's why we write walls of text
.
Of course, the INFP is "organizing" within their inner world. Hence, we build extensive lines of reasoning behind our valuations, something which very few people see, but which the few who do see it can benefit from a lot. We're called "harmonizer-clarifiers" on some profiles because we make sense of human feeling in a way that helps people use it better. Our focus is not application (which is EXHAUSTING), though, so much as understanding. We don't seek to affect people much with our views, unless requested by them, but when we do, there's often a tendency to explain very thoroughly why something is so. We may even be articulate other people's feelings & ideas
for them very well. I've seen other INFPs do this to me even; they see my idea/feeling & hammer it out even more until it's even more articulate. This is useful to anyone who has a hard time communicating their inner world and/or is stubborn & needs extensive proof of another view being possible ( :cough: Ni-dom :cough: ).
The ENFP, however, is the "champion of the underdog"... They get people's attention, they affect people charismatically, they cause change in other people's perceptions of what is what. It's more external, obviously, but less hammered out. INFPs go a little deeper to provide in depth reasons for why these things are important or good or whatever, but they also take less action & affect less people. Sometimes you see an INFP be quite Ne-ish & an ENFP be quite Fi-ish, but we're talking over all personality & tendencies.
. ExFPs go off with eloquent rants that inform people of what-is-what, but INFPs make simple quiet statements and/or ask thought-provoking questions. The latter gets dismissed a lot by T types as emotionalism, whereas the former can carry more weight because it's expressed with lines of reasoning over making people "see".
From an INTJ, that is prized indeed. I'm kinda pressed for time at the moment, but I'm posting so I will come back to it after giving it some thought. 
