The Guardian
Praetorian
Of course the AI doesn't like them... So you may have to protect them with regular units... but the AI doesn't like you to wander in his realm with armed forces...
But they are also very useful to convert the ennemy units inside your territory. Aren't you tired of seeing the AI send escorted settlers to settle right in the middle of your kingdom? Well, just convert them before they finish crossing!
In fact, it's hard to use the monks agressively, as they are hunted by the AI, but they are efficient to defend against "peaceful invasion"
These are all valid points especially considering that hidden nationality and stealth don't work against the AI.