One more time - 5 cases

You have reminded me kindly that there were, in fact, 8 cases in the 
earlier Indo-European languages.  Smyth suggests that they were "lost"
(that is the 3 - ablative, locative, instrumental) in Greek.  
It just seems to me that the world of Koine Greek grammars could benefit
by some clarification on this issue.  It certainly isn't earth-shaking or
anything, but there is a difficulty when the technical grammars (BDF, MHT,
and Smyth) refer to five cases, while just about every intermediate grammar
on the scene utilizes the 8.  
Tom Geer