We value what is rare.
There's a great gag in the new movie "Tropic Thunder," where Ben Stiller's character courageously kills a bear that attacks him in the night--but when he can finally see the bear, he realizes it is a panda: Nooooooo!
But if our cities were overrun with pandas, we'd round them up and euthanize them en masse. The Broward County animal control ALONE euthanizes about 10,000 cats and dogs every year.
Dogs and cats are species that, thousands of years ago, threw their lot in with humans. They collectively saw us as a source of food and protection and stability, and they altered their evolutionary path to become our "pets." At the individual level, this isn't always good for them: people beat them, sometimes torture them, often abandon them or have them put down, frequently spay and neuter them, frequently "farm" them at inhumane breeding operations, and sometimes eat them. Individually, shacking up with humans is a decidedly mixed bag, but collectively they've done better than wild cats and wild dogs, their populations exploding all over the world.
Farm animals strike a similar bargain. If we figure out how to "grow" meat in factory settings, the first effect will be to reduce the number of cows in the world. Eventually they might become a sort of large and lumbering "pet": like a horse. Not needed, just nice to have around. Their numbers will plummet. In a weird way, if we stop exploiting them, they'll be hurt, as a species.
Then there are pests. You've got to admire pests. Okay, I admire them, and I'm fully aware that most people's attitudes toward them is anything but admiration, but it ought to be: pests are the cleverest of them all. They find a way to benefit from the bounty and safety of human society without actually putting themselves at our tender mercies. Sure, we try to exterminate them all the time, but we can't herd them into slaughtering facilities: this is war! They fight back. They adapt. They come up with new tactics. You really gotta admire them.
But of course we don't: we admire what is rare. And one of the defining traits of pests is that they are everywhere--in other words, they are successful! So we tend to despise some of the most admirable species on Earth.
Joshua Klein didn't make that mistake. Realizing that crows must be doing something right, he started investigating them, and discovered that we are utterly surrounded by very clever animals. Watch this great video (which includes video of a crow--spontaneously, without being taught--fashioning a tool: a skill once associated exclusively with humans), and come to see the glory of pests!