Just to put an oar in, wouldn't it be better to eat a piggy that had been wormed with a natural herbal wormer than one that had been wormed with ivermectin based wormer ? I know they give withdrawal time, but who knows what minute amount of residue is actually harmful ?
I read recently that with a new method of analysis, they'd found 14 chemicals in milk (from the pasteurised milk we drink) that had gathered from various sources unbeknown to the producer. I wonder how some of these chemicals react to each other.
I'd stick to natural wormers whenever possible - by cross-grazing, changing pastures, use of herbs etc., which doesn't clear out worms entirely, (that only makes way for a huge influx of fresh worms) but greatly reduces them.
Just my thoughts 
Only if the natural wormers worked to clear already present worms; and the producers could not prove that they did. The other things (cross grazing, low stocking rates, rotation to fresh pasture, access to plants other than plain ryegrass are all things which should ideally be done anyway, to minimise the chance of worms. But none of them will resolve worms once there.
Worm counts then tell whether there is a negligible, low or high burden. Clearing out (which as you say would be counter productive) should not happen as the advice with worm tests is that negligible or low count - dont worm, medium count - worm if you think necessary, high count - you should worm.
If there is a proven heavy worm burden then all the cross grazing and herbs and Verm-x in the world will not address it; only a wormer will do that. If not using a worm when there is a heavy burden the rest of the flock are likely to become worm-ridden too and fail to thrive.
Personally using the sort of techniques we are all agreeing on for prevention, I have not had to worm my sheep to date in 3 years, I would not want to eat meat that has been recently treated with any wormer or chemical and some withdrawal periods seem too short to me! But with appropriate timing of months rather than days or weeks prior to slaughter I suspect we are running greater risks from factors in the environment we havent even got a handle on yet - eg nanoparticles, nictotinoids on bees etc, and there is also an animal welfare aspect of the duty to ensure that we take appropriate action to avoid animals suffering. So if I needed to worm, I would have no hesitation, in the interests of the animals.
