It's a reaction. It's not like you cast it with your regular action and lose your action in the process... you can still attack.
So, the next round you're one spell down, likely still dealt your own damage to an opponent, and anything that tried to hit you is likely several hitpoints and one round down. Which, in a game where combat is based on attrition, means a lot.
Since one of the basic tenets of the game seems to be bounded accuracy, I'd just turn most spell and effects that grant an AC bonus into temporary HP or a damage reduction.