Protectrates count towards ur winning goal so having them is nice.... though they are sometimes hard to keep ... i lost two protectrate when attacking the Macedonians as the Gaul -.- ( had 4 though, Bruttis/German/Britis/Spain... bruttis and German broke away.. costing me to go down from 10 province till 50 all the way down to 27 province till win XD....)

Usually they won't break by themself though, however if u hit their allies they are likely to break ... espically if they are once that are still in the game.

Intersting though, i have never had success getting 1 province factions to become my protectrate... but my highest record was getting the 20 province Brits o_O... ok... well acturally Egypt did once suggest to accept when they were down to 1 province... except that they ask for me to give EVERYTHING back to them.... which is over 10 + province through the Nile up the middle east... ummm no thx ?