I first saw white tigers when I visited Bristol Zoo in 1968. Along with my first okapis, they were the animals I especially wanted to see. As far as I know, there is only attempt to introduce captive tigers into the wild and that is in South Africa (In South Africa, Chinese tigers claw their way back to life - The Globe and Mail). Other than that, zoos keep tigers because people expect to see them in zoos, as they are among the most popular zoo animals; this is a similar excuse to why there are so many meerkats in zoos. They are basically there to attract visitors and make money for zoos. I doubt if there are many people who are interested in tigers and do not know that they are endangered in the wild. There are many endangered species that could be kept in zoos, but aren't.
Basically, zoos have enough tigers to ensure that the tiger is safe from extinction, regardless of what happens in the wild. While I can understand the idea of keeping subspecies pure, the reality is that there will be little scope in reintroducing tigers to the wild, especially in areas with high human populations. I doubt if many visitors are bothered about which subspecies a tiger belongs to and if visitors prefer seeing white tigers to other tigers, so be it. There is a problem with genetic abnormalities, but white tigers are not the only captive animals that suffer from these. Colin Tudge suggested mixing tiger genes (Science: When it's time to call a tiger a tiger: Colin Tudge looks at the complexities of survival for the world's endangered species - Science - News - The Independent) and there may be some scope in replacing tigers with other endangered species that need less space and could be reintroduced into the wild.
Basically, zoos have enough tigers to ensure that the tiger is safe from extinction, regardless of what happens in the wild. While I can understand the idea of keeping subspecies pure, the reality is that there will be little scope in reintroducing tigers to the wild, especially in areas with high human populations. I doubt if many visitors are bothered about which subspecies a tiger belongs to and if visitors prefer seeing white tigers to other tigers, so be it. There is a problem with genetic abnormalities, but white tigers are not the only captive animals that suffer from these. Colin Tudge suggested mixing tiger genes (Science: When it's time to call a tiger a tiger: Colin Tudge looks at the complexities of survival for the world's endangered species - Science - News - The Independent) and there may be some scope in replacing tigers with other endangered species that need less space and could be reintroduced into the wild.