Originally Posted by
butterzz
You are better of with just haste (or mastery)
Not only does haste lower your casttimes, it also adds additional dotticks, lowers the chance you'll get interupted; makes it easier to trow in a well timed fear ( which isn't affected by backdraft) and last but not least, haste isn't affected by rng.
Crit adds to much rng to your burst and you rather avoid rng to much rng as destro since even with good rng you won't be able to kill people in a few seconds and with bad rng you are just screwed.
This might just be a stupid example but I as destrolock ( Only in pvp) with everything reforged can use shadowflame followed by a chaosbolt/immolate + howl of terror before the stun expires while someone that has everything reforged away from haste might get interupted on howl of terror because his casts take to long.