Seeds can and do lay dormant for years in the soil and suddenly appear when conditions are to their satisfaction so althought the pigs may have appeared to clear the ground completetly you'll never truly have clear ground wthout resorting to weedkillers as Hassle has suggested (partial to Grazon myself

) so what you're really asking is it worth getting pigs in to do the job or will you still get thistles, for me the answer would be yes to both as the pigs will fertilise the ground for you as well and at the end you get a freezer full of pork. Where you get pasture you will always get weeds (buttercups and docks are our particular problem!)
Only you can decide if its worth your while to pay for their feeding and well being or to just spray the weeds and not bother with the pigs.
HTH
mandy