OK, lets make up a scenario. Lets say you know an astral mage level 10 that's equiped with 20 astral gems is going to attack one of your provinces that is surrounded by sea. You also know that hes got a pretty big army and that he's got all the intentions of casting master enslave on one of your biggest army. What will you do?
It usually takes what 1 or 2 turns for the astral mage to cast the master enslave spell? That gives you only 1 or 2 turns to kill him before he does... That is not enough time
even with flying units. Maybe if the astral mage was alone and you had flying units.... even then, he could cast it before he is dead...
I've sent a lonely astral mage to cast Master Enslave against a Huge army and converted most of them. and won the battle and gained 40+ units. OUCH.