@DeadlySteve: Since you won't be able to constantly cast nor get the full benefit from extra dot ticks due dispels and the lack of RPPM trinkets in PvP, the value of haste is weaker than it is for PvE. Most of your damage simply comes from your dots and mastery gives the best increase on that damage.
The people saying haste is better are likely mixing up the priorities with PvE priorities.