What makes a class needing Crit or Det or D.Hit in priority? (mainly about MCH)

[Idolon]

Black Mage is one of the few classes in FFXIV that REALLY wants speed above all else because of how our casts are slower than the GCD.

It's not so much the length of casts that makes spell speed good for BLM, but the fact that every damage skill BLM has is on the GCD. Most other jobs do a significant portion of their damage through off-GCD abilities with fixed-duration cooldowns, and skill/spell speed does not affect those at all.