The J/P, by itself, has no meaning other than to indicate the orientation of functions which do have meaning. It's not an extra function that was added in MBTI theory.
Right, function = Jungian, so I can see that. Correct me if I'm wrong, but there is no E/I function either, it's only about the attitude of the function (E = primary extroverted).
Yet E/I and J/P is unquestionably a trait - factor analysis proved that. So, the argument I make is that MBTI is no longer a
theory but an
instrument, complete with the studies to support it (as much as they do, of course).
MBTI is more than jungian theory at an extremely basic level. At this point, it is more the afterthought than the core of MBTI - it is a way to unify traits... but MBTI itself is not the theory, it is the instrument. It even have trait descriptions, including E and J, which are not covered in a Jungian view.
Hence, the concept that they have "no meaning" seems incorrect, at least as far as the factor analysis and everything else goes.
But yes, I know... I'm going to hear about how traits and behaviour aren't cognitive, and that really, there are only 2 functions with I/E attitudes.
MBTI may play both fields, but as it currently stands, their methodology and their instrument says there are four. The theory takes a back seat.