Accepting Protectorate might be worthwhile if you are losing money and are in danger of being attacked by the one making the offer then yea, ask for a bunch of cash and become a Protecorate. And if they attack you right away so what? You just got some free cash. Generally as a Protectorate you simply are giving up your surplus cash each turn to the other faction. If you have little to no surplus cash each turn then it costs you nothing.