I believe the use of an acid hardening fix was mainly used for films where the gelatin overcoats were very soft. In fact some old books recommended formalhyde to toughen the coat (as do some current E6 processes). Packaging I suspect is the main reason for an acid fixer being used for film and paper. I believe Kodak produces the only fixer (liquid) where you get two bottles, one with fixer and the other containing the hardening solution.

I am a firm believer in the use of plain fix (Hypo and Bisulfite) for my prints and films or an Alkeline fix for film. Works just fine and eliminates chances of staining when toning.