No matter how minute the residue is in eggs after worming, (for most wormers and antibiotics,) there's always the risk of increasing worm resistance to the product the next time it's used in poultry. It may require increasing dosage or rotating to a different medication. We see this in the FDA's recent ban on most antibiotics including a couple of good legitimate poultry wormers that were removed off the market.
There are other things that can enhance wormer effectiveness in poultry as well, but most chicken owners are afraid to do it, or wont do it.