In the normal range, however, they tell me which side of the brain is favored when a person is making rational decisions.
I'm pretty sure I said something about this before. But the best example I've got is that we all have two different ways that we deal with mathematical relationships: a left-brain mode, which results in verbal shortcuts (like the multiplication table - 2 x 3 = 6), and a right-brain mode, which allows us to grasp relationships between quantities without specifically calculating (like recognizing that 1,203 + 6 couldn't possibly equal 17).
The left-brain mode allows us to perform precise calculations, which are usually inculcated by education. The right-brain mode allows us to grasp relationships in a way that draws on natural human capacities (spatial logic).
So although the MBTI treats INTJs as though they were rational types, even though they're irrationals in Jung's system, and INTPs as though they were irrational types, even though they're rationals in Jung's system, it doesn't make any difference to me. In my view, INTJs are using left-brain Thinking -- they want to improve the systems they encounter in the outer world, and INTPs are using right-brain Thinking -- they feel a sense of truth in their very bones and believe it ought to change damned near everything.