Zoos can contribute to studies on adaptability, on the behavior of animals in captivity, on animal learning processes, on human/non-human animal interactions, on evolution, and so on in the safe, healthy environments most zoos provide and certainly without ever killing an animal for post-mortems.
Just like zoology is more than just the mentioned examples of etiology, multiple scientific branches have been and are involved when it comes to zoos. You yourself even mentioned one: "human/non-human animal interactions" = sociology/psychology
And they should educate the public about evolution, too.
Try that in some fundamentalist religious countries...
Because zoos are only skimming the surface of each of the other categories you mentioned. In other words, I believe by attempting to be everything at once, they aren't doing any of it as well as they could if they didn't spread themselves so thin. Their focus should be on zoology.
Without the advances in veterinary medicine, especially on the field of zoo and wildlife medicine, your "zoological" research in a zoo would mainly be limited to just doing post mortems. Well, maybe you could create your very own zoo "body worlds" then-like Von Hagens did recently...
http://www.thelocal.de/society/20100318-25973.html
Just as various aspects coexist within a modern zoo, various scientific branches can coexist and even contribute to each other.
I would eliminate recreation and conservation and reverse the order to place research slightly above education.
But, if it is true, let the average layman go elsewhere.
Well, good luck with that; you will end up with a very empty and soon-to-be closed zoo. First of all, a zoo should be at least half-decently financially stable. Without visitors, no money-and thus no education & no research; unless you can find a plausible connection between zoos and global warming to get some fundings...
The thirst for education should be stronger than the thirst for entertainment.
Nice idea in theory, but not rooted in reality.
There may come a day when they will have to be.
I hope not, as zoos neither have the capacities to keep self-sustainable populations for the at least 1,5 million species (and not mentioned hundreds more subspecies, local varieties, parasites...) of animals nor it is recommendable to rely solely on ex-situ conservation. And taking care of injured or orphaned wildlife is (from a financial and logistic point) a bottomless pit with, as reduakari remarked, scarcely any conservation value.
As I wrote previously again and again in this forum, I agree that zoos should not be amusement parks for the idle brainless entertainment of the masses and that the tendency to label all zoos as children attractions (as done by Lonely Planet etc.) is counterproductive for the attempts of zoos to be regarded as scientific institutions, on a par with Natural History museums or botanical gardens. However, you should not neglect, belittle or ignore the importance of the recreation and entertainment factor for the zoo visitors-something also said other scientific have to address nowadays to stay alive. Neither should you ignore the already mentioned important multi disciplinary opportunities a zoo offers for various fields of sciences & art. However, the well-being of animal & staff should be priority.
In my opinion, the amalgamation of zoo, botanical garden, natural history museum, zoo school and even art studio into one institution would be a great idea. The Arizona Desert Museum, to name a prominent example, is heading into this direction.
Zooplantman is right in pointing out the flaws of the "Zoology Only!" & "Ignore the paying public's wishes for their own good" demands. Maybe the nickname "Zoo Visitor" is more apt than originally intented by its bearer, as the displayed point of view and in particular the casual ignoring of the economical, social and (even) political side of zoo reality might only arise from the perspective of a mere zoo onlooker that, no offence, lacks real insight knowledge...