You forgot about worm oocysts in/on the soil and grass that chickens peck and eat. DE is useless as a wormer as the OP attested to with a necrosy report in her very first post in this thread. Did you miss it? If their feet touch the ground, chickens will get worms. One roundworm lays thousand of eggs which are deposited on your soil which your chickens pick up. Therefore you want to keep your chickens worm free as much as possible using chemical wormers, keeping them healthy. Worms weaken their immune system to the point of opening the door for bacterial and viral diseases, causing multiple problems. You are correct about using the same wormer all the time, it causes worm resistance. Rotating wormers eliminates worm resistance. One good example of a wormer that is useless against worms in chickens is ivermectin injectable. It still kills mites in chickens. It may work as a wormer in other livestock, but not in chickens. I agree that fecal samples should be tested. Here's pics of roundworms in a chickens intestine...remember, one roundworm lays thousands of eggs:
