Sorry I missed this thread, and your birthday on the 12th, Horsa.
I do think it's an interesting question, and very complicated. I'm with BB and Darth who say they've very much enjoyed going to zoos over the years. They give us, as children (and grown-ups) a great love and empathy for many kinds of animals, which is useful. And they've been greatly improved in the last 40 years, in that they're not merely in cages, but generally in open and very well thought-out enclosures, which hugely consider the needs of the particular animal. We've come a long way from the PT Barnum days of just taking an animal from the wild and exploiting it. A lot of zoos have animals that weren't born in the wild, and couldn't necessarily live there. They also have the opportunity to breed endangered species, in cooperation with other zoos, taking care to not interbreed, etc. It seems to me that now zoos seem to have much more of a mission to animal conservation, rather than just exhibition, and this is a good thing. Let's face it: when we're talking about how we feel about zoos, we're talking about the large animals. Lions, tigers, polar bears and pandas, elephants, leopards, giraffes, etc. No one gives a toss that a tarantula or a python are in a box. My point being, though, that if you can put those big animals, (and the small ones,) in an environment where they are well cared for, and you can make first-world people love them, especially from very young, you have a better chance of saving them from the poachers and the others who actually would exploit them, or the environmental factors that are killing them. The closer contact we have with them, the more we can empathize and care. That's my best argument for good zoos in the 21st C.