In my opinion, the shadow priest spec lost its identify with the introductiom of void form. Before that, shadow was associated with vampiric/shadow powers that corresponded to "the undead shadow magic" rather than old god stuff. On the other hand void-themed priest is the new interesting direction of the class.
In terms of theme/playstyle separation into two different specs could be a solution for two groups of players: to those who want a more classic version of the shadow priest and to those who like the insanity/void design.
What do you think?