Bah, I tried to get the Egyptians to become a protectorate as the Seleucids, they never accepted even when I had their last city and last army under siege.

But protectorates are great for those out of the way places that you want to add to your victory score and remove as a threat but don't really want to go through the trouble of conquering or holding.

Germany is an excellent example, maintaining it is a pain in the arse and the money you get from there is pitiful, its also full of rebels.

Numidia with its obscene travel times bewteen cities is another one, Spain too is out of the way and once you've already conquered the Gauls its rather useless really as all the action is in the east.

A hint of advice though, if you're going to make a country a protectorate choose any other protectorates with care, for instance if you go through all the trouble of making Germany a protectorate then DON'T make the Britons or Gauls a protectorate, because inevitably your two protectorates will probably be going to war with eachother which means you'll have to end up conquering one of them again after they break the alliance.

Spain and Numidia are another example, having them both as a protectorate is just asking for trouble, unless you capture and keep both Tingi and Cirta and leave the Numidians the rest, depriving the Spanish of a place to attack or blockade, that can work.

It should made so that two protectorates of one nation don't go to war with eachother, or that if two of your protectorates DO go to war with eachother that it doesn't break their alliance with you, or even that you can tell them to stop.