How would you feel if they separated the Priest into "Priest" and "Shadowpriest"?
Imho, Sunwalker Tauren wouldn't be messing around with shadow magic, (Nor would Draenei), and not all Undead and Worgen should be able to wield the Light.
At least, not EVERY priest.
Balance (Ranged Damage: Shadow and Light, for the Pandaren-style light/dark feel.)
Shadow (Ranged Damage)
Psychic (Ranged Damage - illusion/shadow pets, a Mind Control that turns your target into a controllable pet for 8 seconds, replacing their spells based on what they are. Also, a cool effect for this could be City of Heroes style Illusion damage. More damage per second, but puts a HoT on them.)
Darkness (Shadowform + Shadow Orb bubbles, tanking. Or some undead-style healing spec.)
Races that don't make sense with both:
Forsaken are forsaken from the light. They are held together by shadow magic. A Forsaken Shadowpriest completely and totally rejects the Light, as it has rejected him. He would don robes of a human priest, and allow them to be covered in blood and dirt, as an insult to their order. Except, in BGs I constantly see Forsaken Priests in shadowform casting healing spells. There are more Shadowpriests casting holy spells than there are unplayable High Elves in the alliance.
Draenei hate all things dark and wicked. Of course they can be priests. But the only reason any Draenei would turn to the shadow is for the balance.
Tauren priests aren't even supposed to know anything about shadow. They're Sunwalkers, Sun Priests. They worship the god of the sun. Light. And stuff. Why should they be able to turn into shadowy figures with orbs of darkness and psychic beasts?
Worgen can't even be paladins. They aren't one with the light, not anymore. I don't see why the light wouldn't have forsaken them like the undead. They're, for lack of a better word, abominations.
Every other Priest class could keep their priestliness and actually gain a class.
To sum it up:
Priests and Shadowpriests shouldn't overlap, just like Rogues and Hunters shouldn't be the same class.
I don't think Blizzard would ever change this, but how many of you would enjoy playing a shadow priest class, or a holy priest, without so much shadowform?