Hatred should be a purely sustained spec, that puts a good amount of pressure over time. Currently, it has the best of both worlds, in that it has both great sustained and burst... On top of that it has exceptional AoE DoT spreading.
I've been playing Sin/Shadow since beta, and even I think this spec is now over the top. It trumps every other spec in every aspect because it literally has everything...
When they do nerf Hatred/Serenity, they need to rebuffed Decep/Infil. Sin/Shadow middle tree needs more burst, since it got nerfed in 3.0. Now it has mediocre damage at best, but pretty good survivability. They need to take the burst from Hatred/Serenity and give it back to Deception/Infil. Then it'd be in a better state, both specs would be playing the role they are suppose to be playing... Decep/Infil as burst, and Hatred/Serenity as sustained.