I don't understand why so many people are saying this, it's completely wrong.
Haste is still better than mastery on AoE heavy fights due to PPM, and infact crit is better than both. Mastery is the weakest stat, presumably your opinion is an assumption that since you GCD cap CL and Crit is "weak" single target, Mastery must therefore be better, which is not true at all.
Totemspot 5.3 guide to stat values in DPS on different styles of fight. Admittedly, next Wednesday this will all be somewhat wrong with the Haste/PPM nerfs, and we WILL be gemming mastery. But for now, Mastery is arguably your weakest stat on Horridon/Durumu, Haste's decrease in value isn't as large as you think, and it's actually Crit that pulls ahead.