I looked. I don't gain anything. The only thing I get is a useless ally. Carthage is really, really weak at that point in my game. I experimented with a more powerful Spain, since they still have around 4 provinces, and I managed to make them agree. The advantage of having a protectorate, I suppose is gaining an ally to everything that I do while not worrying about that faction allying with the other side. It's actually not much of a bonus since the Senate made me grab Carthage before that and the Scipii already had 2 of their provinces. They're just cannon fodder now.