Why do people commonly refer to Na2HPO4 as Sodium Hydrogen Phosphate, rather than Disodium Hydrogen Phosphate? First of all, I am aware of this question.  The purpose of my question is to understand why people seem to break the rules that IUPAC has set (See example five on page 76 of the book), especially since this is not a compound that has a historic name, like $\ce{NH3}$ being ammonia or $\ce{N3-}$ being azide.  This compound has two species of cations, with sodium having a 2:1 ratio to hydrogen.  According to example five on page 76, compounds that have multiple cations are supposed to give prefixes to every compound in an ion that has a quantity greater than one.  Why do we not do this for sodium hydrogen phosphate?
Well, turns out that I needed to look at this a little differently.  I looked at calcium hydrogen carbonate and then realized that hydrogen is part of the polyatomic ion.  It's a bit harder to recognize when there are no parenthesis, like in the case of sodium hydrogen phosphate, but the same thing applies there.  Therefore, the name, sodium hydrogen phosphate is correct because there are still only two distinct ions in the compound.