Shadowform was supposed to have Sw:pain scale with haste but now they removed it

Quote from Blizzard staff
Priest
Shadow Word: Pain and Haste in Patch 3.3
We removed Shadow Word: Pain from scaling with haste because we thought Shadow dps was too high with all 3 dots hasted.

There is a bug where you can get big SW:P dots and then keep them rolling at that magnitude forever since the spell gets constantly refreshed. It's a nasty bug to fix. However, that isn't why we removed SW:P from Shadowform.

It's important for classes to want the stats that appear on their gear, but it's not a goal that everyone requires all stats in the same 1:1 ratio. It's a bit unfortunate in the best-in-slot-or-nothing mindset of some of our players that the second and third best stats sometimes get labeled as junk even if they still boost dps beyond a non-trivial degree.

Our stance on simulations remains that they can be good tools when used correctly. We will continue to not pay a lot of attention to posts that simply say "Sims say our dps is too low. Please buff." You can't just accept a simulation's estimate of your dps as what you'll actually see on an encounter. Sim output is fundamentally not empirical data -- it is a model; an attempt to imitate what will really happen. Simulations can be really useful for predicting say an optimal talent or gear configuration for your character. Players have learned a lot of things about class mechanics from the better sims out there. "True dps" is not one of them.