Just like everything else in socionics, it's apparent in some people and not in others. Some people fit cleanly into one of the four slots. For some people it's only apparent that they're Ni/Se or Ne/Si. For some people they don't seem to fit into any, and some seem to fit into all of them.

For me, it's helpful in understanding why some partners have been boring or overbearing to me (Ne/Si). It also helps me understand why in spite of many mutual attractions to Ni types, I have not ever actually done anything with any of them. *shrugs* If one of the attitudes makes sense for you, use it when it applies to a situation you're in. If not, don't. And be careful to avoid over-analysis.