The only good thing I think Zoos ever do is excite young minds.
Otherwise.. I think its cruelty to animals. Even great zoos like the Pittsburgh zoo that are 'cageless' and try to really provide natural habitats as much as possible for its animals, and even though I think the teams of zoo keepers really are trying to do good, Giving a lion 50 pounds of beef is NOT the same as letting the lion have the satisfaction of its own kill.
Saving an animal from extinction in a zoo, really isn't saving the species, its just saving something that once was the species so we can gawk at it.
mj