When you can anticipate and stop attacks/spells 8 out of 10 times. Avoid several strikes a round for the cost of a few mana. Then avoid the other in between round attacks that you aren't anticipating at about a 50% rate. All while dropping massive melee damage and wrath spam? Something is wrong.
And don't give me the "they can't lag you" argument. I get that they have no sealing ability alone. It doesn't mean it makes sense to have a sub that is damn near unkillable in most situations. I don't know if people played monks like shit when they first came out or if this is a product of a recent change but there are monk paladins everywhere now.