To quote a former WoW developer,

Does Blizzard worry most about dps balance as raid tiers open, and not that much about balance as ppl start farming the tier?
Can't speak for it now, but when I was there, we worried more about new content than farmed content balance. (OccupyGStreet)
The idea being that a low performing spec might get sat on a new boss, but unlikely when stuff was on farm. (OccupyGStreet)
At some point we as a community started worrying about an individual's "right" to high DPS rather than the DPS of the entire raid. (OccupyGStreet)
Rather than "Did we beat the boss?" it became "Did I beat my warlock friend?" (OccupyGStreet)