First let me be clear that I do not support mixing. However, logic must prevail when someone asks why. Would there not be the same risk with putting two frogs of the same species/morph from different vivs together? Any two darts frogs that did not grow up together could exchange parasites or diseases. There is no greater risk of this, that I am aware of, between species that between two Green Sips or two Leucs, etc.
If I was new to the hobby and was given that as the sole reason not to put two males of different species together, then I'd be confused. As I understand it, a logical reason not to mix, even if it is safe for the frogs and no breeding will occur, is that it can confuse others new to the hobby into thinking it's ok when they come to your house and see the viv. Most people automatically think of it in terms of a fish tank. That is why zoos that display darts in mixed tanks do not help the hobby IMHO.