It's better to have people view the gorillas and learn about them than have people slaughtering them. As Professor Roufs stated, mange will make the gorillas look unattractive, but otherwise it won't harm them. I think it is good for the gorillas to live in their natural habitat.
Most people visit the zoo's anyway to see wildlife.
Only a select few would go to see them in their natural habitat. In their natural habitat, it is a way to teach the people how to better take care of the animals. They are taken care of a lot better there than they are at zoos. It is a safe refuge for the few gorillas which are left. I agree that the people viewing the gorillas in the wild are not the same people going to the zoo.