5 or 8 cases

As to the number of cases, A. T. Robertson himself discusses this matter
at length on pp. 247-50.  Robertson relies heavily on comparative
philology for justification that "the usual seven (eight with the vocative)
Indo-Germanic cases are present, though in a badly mutilated condition
as to form.  The ideas, of course, expressed by the cases continued
to be expressed by the blended forms.  In actual intelligent treatment
[!] it is simpler to preserve the seven case-names ..." (p. 249)
I do not quote this as the final word on the debate, only to address
your question of why 8 and not more cases.  According to Robertson,
at least, it was not function alone that characterized case, but
form as well, as attested by comparative philology.  

Gary D. Collier
University of Denver/Iliff School of Theology