What I have noticed in the ESFP type is what I called "intuitive runaways" where conclusions are drawn from thin air (or more likely, based on the immediate Se) ... which are wildly inaccurate and generally hilarious. The best part is they believe them, like they "saw" the future. Hehe not like tertiary Ni is much better...
ISTJ - well it's hard to give an unbiased opinion, but what has been said about resistance to change and envisioning potentials for disaster certainly strikes me as very accurate.
Myself as Ni-tert., well I know you haven't specifically inquired about ISFP but the thread title does generally ask about tertiary N. I find I have little use for Ni and that I prefer to rely on information that is readily available (wow talk about a textbook definition?). I wouldn't outright say my Ni is "weak," but I don't really rely on it and am not really envious of those with Ni in a higher-up position.
I find that tertiary Ni has a lot to do with supporting the dominant function. Explanation: while Se is the preferred "information gatherer", the data that Se draws in is also constantly processed via Ni. As the role of the auxiliary is to support the dominant, the tertiary fills in the blanks to a higher degree, and a more complete representation is accessed. A concrete example of this is that I generally find my Ni function being used most effectively to imagine scenarios involving people, and basically "seeing it thru to the end" before I even start down that path.
Remember MBTI indicates preference and relative strength, not absolute strength. I personally believe that my Ni function is quite powerful and deep. Most definitely, I have the ability to "extrapolate from incomplete data", in fact I love doing so, but preference is preference and we can't help it.