You rotate the wormers so as not to build resistance to all the drugs at the same time. Treat worms with one type and any resistant ones can be removed with another type of wormer later. Always changing it up keeps the populations down; the worms that survive one type of drug will continue to reproduce only to get wiped out by another type of drug they have not built resistance to. Rotation I think is the key to healthy livestock and I don't know if all the drugs present at one time have any negative results or would be toxic when mixed together. Too many variables??