"Pro hominem" is rather doggy Latin, certainly (although there is some overlap between the accusative and the ablative beginning in late antiquity). The phrase seems like a personal (and rather recent) idiosyncrasy, rather than a term in general use.
Are you sure? I can't find any such usage in my database of medieval latin, except for
Paulo inferius cod. aa generalius, sed minus conformiter dictis Aristotelis loc. cit., mundum pro hominem.
But this is saying that the word 'mundum' occurs in the place of 'man', which is different. (And it is equally acceptable in medieval Latin to decline even quoted expressions).
I think the reality is that it is a personal (probably Wikipedian) idiosyncracy. Certainly this article http://thenonsequitur.com/?p=84 , which mentions the argument, uses the apparently correct 'pro homine'.
It's not merely a Wikipedia fallacy, as googling "pro hominem" reveals it used at a variety of other sites. (I assume they didn't all simply copy the phrase from Wikipedia.) I would guess that many people independently formed it by half-assed analogy (instead of by Latin grammar) from the better known "ad hominem".