I remember Gerald Durrell saying that he would prefer it if the world didn't need zoos, as this would mean that there were no endangered species. Unfortunately, many species are endangered and zoos have saved a few from extinction, but this is a very small number compared to the number of species that have been kept in zoos.
I agree with mazfc that it is better to spend funds to preserve animals in their natural habitat. That was one reason why I was upset during London Zoo's presentation to encourage people to pay towards the £3 million 'needed' for Tiger Territory. Of this £300,000 would go towards helping preserve Sumatran tigers in the wild. The rest would go towards a big enclosure for tigers at the zoo. If the figures had been reversed, this may have helped save Sumatran tigers in the wild. There are about 250 Sumatran tigers in zoos and this subspecies has been saved from extinction. I joined the RSPB because of it paying towards preserving habitat in Sumatra. That is true conservation, not an expensive new enclosure for captive animals.
At least Damien Aspinall is putting his money where his mouth is and using it to send captive animals back to the wild. Whether this will be successful, I don't know, but zoos should be doing a lot more to keep animals in natural conditions to replenish natural populations. I get a bit tired of some zoos talking about conserving endangered animals while a large percentage of their stock are 'Not Yet Threatened' or are common in zoos and have already been saved from extinction.
Zoos should be looking more towards saving smaller species that could survive in small areas of natural habitat, rather than keep on promoting large ABC animals that would not survive in the wild, even if there was enough space for them. 'Meerkats United' showed that a relatively small mammal can become popular (too popular?) and there are many other small animals that could do with being promoted in order to save them from extinction. Hopefully 'Nick Baker's Weird Creatures' has promoted some of them and marketting departments at zoos should look beyond current crowd pleasers and accept that other species could be just as popular.