Originally Posted by
Hctaz
It isn't a matter of whether or not you do well in your raid. That isn't the evidence we're looking at. Take a look at warcraftlogs' data. They have a LOT of data from a LOT of different raid groups. You can see how every spec compares to every other spec. You'll see that frost is very low comparatively, especially when compared with unholy.
Unholy is the best melee dps spec in the game right now, and the 4th best overall spec in the game. Frost is only better than a handful of melee dps specs and only better than BM hunter as a ranged spec. The problem is that Frost ends up being worse than every other class except for demon hunter and monk. It's better than Survival, assassination, and outlaw, but hunters are all MM which beats Frost and rogues have Sub which is extremely powerful due to their class being OP at soaking mechanics.
Unholy also generally has better defensive traits. Better AMS/IBF and a free 20% avoidance is very powerful. Frost has Mirror Ball which can be an advantage, but only really shines in Sister of the Moon. In every other fight, you can either preemptively use AMS to avoid debuffs, or play well. Sisters is the only fight that has a magic debuff that is randomly assigned and is a big pain in the ass for the entire raid if you clear it.