While I strongly believe in that also animals know emotions, I as strongly believe that the feeling of being demeaned by having their photo taken is not among them.
Yes, I have taken photos of animals in cages.
There's just so many animals I would otherwise never ever have seen with my own eyes (and camera) in all my life.
Zoos are much better today than they were when they first started, when animals were captured and put into really narrow cages for people to look at them (in pre-TV times, when to see a tiger in a cage was THE event in town, for example).
Today, in most zoos all over the world, large attempts are being made to a) keep species alive that are on the brink of getting extinct in the wild, b) give the animals the most natural habitat and as much entertainment as can be given.
The fact remains that the animals are NOT in the wild.
Yet, out in the wild (and Mike_E points that out well) it is much harder to control what man does to their habitats, and man can wreak true havoc to many animal /whole species out there, as we also all know.
So while the (blooper) photos of the tigers in a zoo where I went to last year in summer (
in_this_thread_here ) can only serve as one example of how things should NOT be, but definitely NOT, other photos of caged animals can be one of the motifs you choose to train yourself and your photography... either by trying to totally blend/blur out the cages like is shown in the example photo above, or by working with them as means of your composition (like I did
here - just because I had to).
I do like photos taken in zoos that actually manage to make it look like they were taken in the wild, though I do feel it should be TOLD where the photo was taken (so far all our members have done so, I think).