Right off the back I want to say it is good. Actually very good. This new practice will only become better and more empowered by SMTs. It gives everybody, every niche, every idea a specific place/forum to freely relish in what they feel comfortable in. Also, an added benefit is the double rewards in forms of tokens and STU.
I also had the thought about too many cooks spoil the broth. But lets think about the bigger picture. It is already providing each interest a specific forum, so that means more non-steemians can come on steem and not feel over-whelmed by the craziness that happens on the trending or highlighted sections of steemit dot com. Which is a good thing.
Second, each tribe has to be powered by a strong community. Look at sports talk, if they did not have such an active community who are very passionate about sports, they would have dissolved long back with no value to their token and their use case not being implemented well. What I am trying to say is if a tribe is useless and it's community starts thinning, they will automatically dissolve. Similar to natural selection...but for tribes.