My opinion is that huge tribes are more of a problem, not something the game should encourage. From the point of the long-term success of the game it has to be attractive for new players. If there are huge (500-1000 members) tribes, a new player will never get the opportunity to form a competitive tribe. And if the big tribes don't accept newbies (which happens pretty often in games), a new player would feel that he has no chance to get the same game experience as the veteran players got. Its a bad game design when the players who join later can't have the same options as the veterans.
If there was a population limit for tribes, something about 50 members, anyone could form a new tribe and wouldn't be hopelessly lagging behind the old ones. The gameplay would be more diverse too, more alliences, more conflicts, more trading. When a game is ruled by 2-3 big clans then it usually starts to get boring, and on the way to die.