Problem with an evasion tank... if they are any good at all they're going to replace PLD and WAR as the go-to most likely. If you have bosses that can already almost one-shot tanks if they aren't prepared for it, how would an evasion tank deal with it? They can't have the same high-HP and self-healing as a WAR, and they can't have the raw mitigation of a PLD, and if they can just dodge those attacks why bother bringing anything else?