Overwriting is the least of the worries in my experience.

A baby AST to me is the one when they enter the instance and spread their Ewer card. You know where this is going.

That card sits spread for 75% of the instance. Any card drawn from this point and used is bare, and while you as the Black Mage cry inside for some arrow action to pump some more spell speed blood in your soul to unleash the burn on the boss, they just stand there with their cute 0 DPS.

Eventually they use they use Ewer on themselves, or maybe not. But don't worry another Ewer will get held..for themselves.