Interesting. Some of the choices Blizzard made are pretty obvious -- for high end Arena play, a crab probably isn't going to do much for you. But Mania's big problem is with Blizzard including the spider family, and excluding the gorilla -- spiders don't bring too much extra to the table, but gorillas bring Thunderstomp, an AoE damage effect that isn't necessarily overpowered, but can help in some cases. There may be other factors at play here -- 15 sounds like a round number that Blizzard may have had to adhere to, so maybe that's why the others aren't included.
And if nothing else, as Mania says, making these choices might encourage Blizzard to look at some of the weaker families and beef them up a bit, either for endgame or PvP. And all Hunters, no matter what pet they choose, will probably appreciate that.