SMN needed a nerf. Plain and simple. It was doing more damage than SAMURAI in the 95 percentiles, being top dps. And it brought radiant shield/contagion & devotion. The BLM buff is going to add around 220 dps based on 95 percentile again. This will even out casters as is should be BLM > SMN > RDM, but will also most likely destroy SMN's already fragile place in any sort of meta.

On the other end they buffed MNK so much that it could replace MCH in the meta in triple melee friendly fights.