Although it is very rare, they do occur naturally in nature from time to time so I don't think it's entirely inappropriate to exhibit them in zoos. The problem is too many people want them so they have to inbreed to produce an unnaturally high percentage. If all zoos could agree to introduce one or two white lions (or white tigers in the case of indian tigers) and let the genes float around and periodically produce white offspring like in the wild, I think it would actually be a good idea. Of course that will never happen, because those who are not lucky enough to get the occasional white cub that pops up will be jealous and others will always pay a premium to bring in white ones. From a purely aesthetic point (ignoring the ethical implications), I do think they're quite beautiful.