The biggest danger is the one HBD mentions - encouraging the evolution of bugs that you can't kill with anything short of two bricks, or weeds that you must pull out individually. I'm not worried about eating "frankenfoods" nearly as much as having no food.
If the grasshoppers eat all the barley, where will Guiness come from?
I edited post #4 to include weeds - it's just whether the pest is in Plantae or Animalia, anyway. And yes, we can go back to hoeing cotton and smashing boll weevils, and perhaps help unemployment in places by doing so. But you should see some of the cotton fields and their associated dust storms out here in the High Plains of Texas. No-till cotton farming actually captures Wal-Mart sacks instead of letting them blow to Georgia - and holds most of the dirt that used to accompany the sacks.
Having extra dirt in Texas doesn't seem to be an argument against that.
No, it's an argument for "Roundup Ready" plants - no-till is way less wasteful of both dust and diesel than traditional farming. I just share the OP's concern that when we breed herbicide/insecticide "immune" pests, we will be in some degree of trouble - like we are with multiple-drug-resistant tuberculosis.
I guess I don't mind sharing some of our dirt, but dealing with all of it that fails to escape and blows into the house every spring is a pain in the butt.