I think a zoo or safari park is a far better place for seriously endangered animals, such as some species of tiger where there are only a few hundered left. If they were left to their own devices in the wild some animal species would be extinct by now, either by poaching or illness/injury.
Fair enough, animals bred in captivity very rarely return to the wild because they don't have the survival skills and instincts to cope. That means that eventually endangered species will only exist in zoos but if it stops them from dying out altogether then I'm in favour.