My take on purging is simple. If you get crawfish from a seafood house and they are already divided in size, they have already been cleaned and in a cooler for awhile. I don't think purging is needed as long as there is not a lot of trash visible in the sack and the crawfish look clean.
If you do like we do, we get them from fisherman or run our own traps. We always purge and you can taste the difference. You can also see the difference in the boiled water itself. I understood it really doesn't make them poop as much as throw up. It gets evrything out of there stomachs that would wind up in your boil. This is also another reason we boil the side items before the crawfish and not with them. This is very true for crabs also.

Rich