As the Unity 5 beta already uses the specular workflow, I doubt they'll get rid of it. I think they're just trying to determine if they should also include the metalness workflow.
Actually there is : simplicity. People are already confused enough over what PBR is and what it implies. Arguably, both workflows have only very minor advantages over one another, so it makes sense to just stick to one. Having both means having double code paths for Unreal/Unity to maintain (for their shader and everything…