Zoos can and do provide an invaluable 21st century need, providing education, species rehabilitation and growth, study and more. Without zoos, we'd be left with nothing more than Discovery Channel or PBS documentaries showing something on camera (and pledge drives), providing no immediate, visceral emotional attraction and hopefully interest and involvement in the preservation of various species. In the end, we'd likely lose more than we saved. A well-managed, responsibly operated zoo is critical for promoting and preserving as much wildlife as possible.
Growing up in San Diego, I know more than a little about the topic, and watched the San Diego Zoo grow from an early-century "monkeys in a cage" venue into something that, while still open to improvement, gave hope that there was somebody doing something that gave some promise for the future. That institution in particular, attracts some of the finest minds in the field to work and study; there are Animal Husbandry and Veterinary Science PhD's shoveling poop in the enclosures just to work there.
Face it, unless most of the public can see and hear and smell these incredible beasts "in the flesh", they're less likely to really care enough to endorse, promote, and demand protection and sensible management. Most of my concern for the wild came from my thousands of trips to the zoo and developing--at first--a purely emotional connection, then continuing to want to know and learn more, and moving on to endorsing their care more strongly than I would had I only seen "March of the Penguins".