Quote Originally Posted by Gormogon View Post
BLM benefit so much from spell speed. Moving now isn't a major loss, you can cast thunder 3 instead of 1 or 2, and doing a triple - quad flare rotation isn't so time consuming that it becomes a loss in dps.
The tier of Thunder you use is determined mostly by your MP entering the umbral phase, not your amount of speed though. Viability of Flare remains constant as speed increases, since all of your skills speed up by the same proportion.

The reason speed is so good on BLM is mainly because the usual king dps secondary stat - DET - is poor on casters due to their high base damages (mainly due to the very high weapon damage). BLM also derives most of its damage from non-DoT on-GCD damage, which speed benefits, and doesn't normally autoattack (which speed does not benefit) - therefore speed benefits the vast majority of BLM damage.