That's a very good point, which I think is where some of the trouble comes in for new classes at all now.
That logic could be applied to Warlocks and Necromancers as well - people like
Kel'Thuzad and
Helcular started out as mages, but then started to become Necromancers later on. They're both considered Warlocks as well, through Helcular's spells and an
armor set for Kel'thuzad. I'm not advocating that Necromancers would become a spec for Warlocks or Mages, because in my opinion they're quite different - but there are plenty of people do do feel that they're too similar.
Likewise, with Dark Rangers, they're certainly not like other hunters. We see them use dark magic, summon dark beasts, sometimes turn into spirits, among other things. But, you have a really good point - they started out as hunters and became just a dark hunter, utilizing their new forms/powers with their old skills. That's certainly unlike all the other classes added so far, where DKs were any class before they became a DK (like Koltira, being a ranger), Monks were trained that way, and Demon Hunters were all sorts of things before (Vandel was a rogue, Jace was a mage).
So, in the end, if they were to add a Dark Ranger, I would assume that the entire class would be a hybrid, like Monks. Instead of just a Brewmaster, like people were speculating, they gave them the generic name "monk" and made Brewmaster it's own spec. Perhaps they could make a spec that is essentially a Dark Ranger, and another spec that is essentially a Shadow Hunter. Overlap between the two (and other classes), but they are different enough to be their own specs. They're both somewhat of a Hero Class on their own, fitting with DKs and DHs. Likewise, it would give bows/guns/crossbows to another class, as well as glaives.
That said, I wouldn't mind if Dark Rangers were just a spec for hunters as well, but the problem is that we don't exactly see living Dark Rangers.