Regardless of the proto-history of the Greek language, by the time of the earliest extant remains of Greek these cases as formally distinct are at best only barely traceable. By the time of Hellenistic Greek the formal categories are restricted to four or five distinct inflected cases. I don't have it at hand to cite at the moment. Rodney J. Decker Calvary Theological Seminary Asst. I'd like to offer my opinion on this subject.
Inflections evolve naturally as an integral part of a language.
Categorizing is an empirical definition of these structures as observed in use. We use categories as tools to help us discern the meaning of a word in context. Later, I was introduced to the four-case system. The four-case system, conforms to inflected forms, provides a broader, fundamental description of each case. The eight-case system appears to define the eight most common specific usages, but it can cause difficulties in interpreting instances that don't fit precisely into one of the eight.