number of cases in PIE

Hello, all:

I was wondering how many cases there really were in PIE, or more specifically, Late PIE.  I specify Late PIE because according to most information I've read, PIE went through an earlier "ergative" stage first.  In any case, most sources seem to agree that there were eight cases:

nominative, genitive, dative, accusative, instrumental, ablative, locative, and vocative.

However, Dr Letas Palmaitis argues any cases other than nominative, genitive, dative, and accusative are innovations for that particular language (group).  He claims that Prussian, and by extension Sudovian/Yatvingian, maintain the archaic PIE four-case structure.  He claims that where one case might have a particular function in one language group, that function will be taken over by (a) different case(s) in other language groups.  He doesn't seem to take into consideration that the languages most closely related to Prussian, Latvian and Lithuanian, and then the Slavic languages have, or had, at least seven grammatical cases.  If you're interested, you can read his argument at http://donelaitis.vdu.lt/prussian/reconstructions.htm -- it's the section labelled "THE 11th PRINCIPLE OF THE RECONSTRUCTION".

I have a book, in German, on Prussian grammar (Altpreussische Grammatik by Jan Endzelin) that indicates remnants of the instrumental and locative cases.  So, it seems to me that Prussian might be the innovator in dropping the additional cases.  MIght that be because of early contact with Germanic?

I would appreciate any clarification on this.

Thank you very much:

Andy Howey