The problem with casting a spell in this game, it completely remove chances of blocking and parrying if you take damage during the cast, so its bad to actually be casting for any physical moves generally.
For tank buster like A4 and A1 savage that are magical based, casting spell to survive these moves are still a viable way to deal with it since parry and block are useless, are still useless since these moves would still have a high chance of stopping you when you try to bait a heal after the hit lands on you.
Also, even as an OT, you would need to use them when you are taking damage, think A4 orbs and small room add, and yet you have a high chance of getting interrupts from the damage.