5 case

I've got to admit...I'm a little confused here.  I thought the message
I sent went only to Gary but David captured part of it and asked about it.
I meant for it to go to the group, but I hit reply instead of send.  
But on to the point...
I am not dogmatic about it at all, but it seems to me that the 8 case
system for Greek is a little artificial.  Blass/Debrunner/Funk use only 5;
H. W. Smyth's Classical Greek grammar refers to only 5.  There are, of course,
only 5 forms.  There are just as obviously more then 5 functions.  My
question is why we would stop at 8 cases and not instead use 25 or 30?  Of
course that would be too unwieldy, but if function determines case, why not?
I wonder where the 8 case system worked its way into NT Greek grammars.  I
suspect it is primarily because of A. Robertson's massive grammar, but I
don't pretend to really know.

Tom Geer