Why are five pin XLRs used for long MONO audio connections?
There would be no significat reason to use more than three pins  (as all mono XLR have).
  One for minus, one for ground, and one for sound (+).

I believe that it was used just the same way as (for example) some guitarists would use stereo 6.3mm (1/4inch) cable for their signal.  In principle yes, they are getting "better" signal because the resistance of one pin of the cable is somewhat lower.. But then again - a good cable will have a really low resistance (impedance) to begin with.

To sum up, there is no advantage except for one thing - in theory it is possible that such a cable could be used for longer distances. (Perhaps that's why they are sometimes being used?)  But anyone would be better off to use the appropriate tools for longer distances.