I love Monty with a passion... and it's not only that deliciously fruity headdress. He's eager to fight, and he remains backwards... a good combination. If your personal little maniac techs too well, he might become a threat eventually; no real danger with Monty. If your personal little maniac actually wages war in an effective fashion, he might become too powerful for your convenience - again rarely an option.
Montezuma is very good at dragging another civilization down in a series of pointless wars, which is exactly what I want in a henchman. It's the unpredictable, backstabbing variety that gives me trouble. Give me a hardcore warmonger any day; it's the likes of Catherine and Gilgamesh that make me uneasy.