The argument made during the MoP Beta and during MoP itself was a question of organic assessment. The presence of Alliance and Horde Pandaren on the field of battle forced a reliance on the UI elements of the game as opposed to a more organic and spur of the moment visual assessment. UI configuration and set-up is also wildly variable in WoW due to the abundance of add-ons - further reliance on externalized UI components for FoF assessments weren't received so well by either the PvP or the modding communities.
The Void Elf race may be an attempt to resolve these issues by creating a similar race for the Alliance without having it be the exact same race as an existing Horde one, relying on more visual and thus more organic context clues for FoF purposes.