I've been talking with someone via pm about zoos also....so, I'll say the same thing here - zoos, well run zoos, so serve a valuable purpose.
I have mixed feeling about zoos. If they are well run, they aren't cruel and they serve an important function that people don't realize, in that they preserve the gene pools of many endangered animals, and have even successfully been able to reintroduce certain species back into the wild. There is often little we can do in the short term to halt environmental degradation and habitat loss, which endangers many species - or poaching which threatens the tiger, elephants and has now added the black rhinocerous to the list of extinct mammals in the wild. Zoos allow people to research species and their behavior in order to better understand how to protect them, or breed them for eventual re-release into the wild. The other side of zoos of course, is the public display. A well run zoo is highly conscious of balancing the need of the animals for "privacy" vs the need to be able to have the public see them, and see for themselves what they are like, in a simulated natural environment. That's the hardest part, but I think it too is necessary. There is a difference between seeing a picture of a tiger, and seeing it in the flesh where you can get a true sense of it's majesty.....
as the poet Blake put it
Tiger tiger burning bright
in the forests of the night
what immortal hand or eye
could frame thy fearful symmetry?
When people feel that connection, then they can also understand why it's so vital to save them.
Unfortunately, sad to say - the nearest zoo to me now is a dreadful little "wild animal park", a roadside zoos. Those do nothing to help the animals and are frequently poorly run and cruel.