Ne and Se have a great deal of overlap. They perform similar tasks in different arenas; both prefer to learn through a purely experiential approach and reject the ideas of planning ahead, feeling that "jumping in and getting your hands dirty" is the only way to really learn anything.
Ne and Se are both focused on the external environment and require validation from the environment in order to make sense of their perceptions.
Ni and Si share none of these properties and are focused merely on one's own internal perceptions of the environment, be it the literal or theoretical one. Neither Pi function relies upon the external world for information and both are totally closed to external validation/invalidation of their ideas, and neither is willing to leap into any new situation unprepared because neither knows how to respond to its environment without a period of preparation and internal consideration.
Once again, it's the direction of the function and its Perception/Judgment quality that matters, not which type of Perception or which type of Judgment.
Why do you think ENTPs are so much more similar to ESTPs than to ENTJs? They share Pe+Ji; ENTJs are a totally different animal with Te/Ni. The way they prioritize everything is completely different, and yet they share Thinking and iNtuition with ENTPs.
The more important difference is that they extrovert the Judgment function and introvert Perception--no P type does this and it makes a dramatic difference to their internal motivations.
I do not use MBTI, nor do I believe that there do not exist people who have functional orders outside of its 16 molds.
I simply believe that such people lack skills vital to successful interaction and typically suffer from personality imbalance because having E, I, J, and P functions at the ready makes one most adaptable and most able to handle the greatest number of situations with the best strategy for each.
I trust that I don't need to explain what happens when one lacks a strong E/I/P/J function.
People with two introverted functions can often improve their results in life by working to catch up the extroverted function that Jungian psychology predicts "should" be the auxiliary. It should be intuitively obvious why this is, and why it's also true if you rely on two extroverted (or two perceiving, or two judging) functions--you are ill-equipped to handle situations in life which require the opposite type of ability.
By the way, cut the crap about no consistency of thought. I'm really getting pretty tired of you Jumping to this ridiculous conclusion before you've bothered taking the time to grasp my ideas. Quit whining.