Descriptions of human behavior should be extensible and reducible. Why?
I would say that in the present context Wilson does not use his ad-hoc criteria to differentiate two types of models of human personality – those that fulfill his criteria, and those that don't –, but that this is a pseudo-argument, legitimizing his momentary disregard of the model's complexity and allowing him to focus on only the four basic dimensions in his argumentation.

I call the criteria "ad-hoc", because he appears to make them up on the spot and not motivate them in any way. In my opinion, the whole book is filled with this kind of normative setting of dogmata.