AI protectorates are passive, but AI protectors are backstabbing cretins. The only reason to agree to become a protectorate of the AI is to get as much money as you can from them in the agreement. Often the AI will attack you the same turn immediately thus ending the alliance, and wasting all their money. Which is fair enough - if they're going to be idiots, punish them.

I really hope this feature is improved in M2TW. it's a great idea, but handled laughably in RTW.