I think it's okay for them to have rare, endangered species. They can research them and breed them. I think it is also ok for them to have injured animals, or rescues of the exotic pet trade that probably wouldn't live in the wild. Some of the cages, though, are very small.
Also, there are good and bad zoos.
I have seen some zoos where a tiger was kept in a concrete-bottom wire cage that was about only 3 times its length. All it did was pace around- that is a bad zoo- I will never go there again.
I have also seen big zoos where tigers have lots of room to roam and play. They were in a large space that was planted and decorated to make them confortable, and look like their natural habitat. That is a good zoo.