Permanency is almost never a good idea with a game like this. Sure, it can encourage people to commit, but to what degree? If they really hate being in that pride or pack, they can just stop coming online with that character. And so many things can go wrong. What if you join a pride and it turns out it's run by a jerk? What if the members are rarely online and you end up stuck in a pride only a few people actually participate in?
Not even real-world marriages are set in stone for eternity. It seems a bit much to expect people to stick with the the pack or pride they choose as long as they want to play a particular character, through thick and thin, no matter what happens. It's just not realistic.
Besides, if people really want to leave a pride, they'll just delete the character and recreate it. And if someone is only in a pride because they can't remove themselves from it, what exactly has been gained? They're stuck with a bunch of people that they may not like in a roleplay they may not enjoy, and they can simply ignore the pride, all while still being stuck on the pride list.