Having a rule for ability use (Can't be used while casting a spell) means having exceptions makes them that much more notable (CAN be used while casting a spell).

Iaijutsu didn't need to be a cast time GCD, but doing so on a class that otherwise has nothing but instant on the move skills sets a certain tone and weight within it.

Black Mage's unique ability / abilities can break the rule. They -could- be used while casting a spell.