Imagine having to get snap aggro and having to wait 2.5s (or more) for your cast to go off. Meanwhile, your co-tank that just ate a vuln-up buster takes an auto and dies. Or, you're MT and a DPS pulls hate, so you use your aggro spell and 2.5s later (or more) you get aggro, but your DPS dies. No thanks. I'd rather not have a caster tank beyond what PLD does with HS.