Perhaps your cousin's vet was not quite up to the ethical standards that other vets pursue? There are always going to be exceptions and there are always going to be those with lower standards than the main body of a profession. There have been many doctors who have provided unnecessary medication to people. Some examples that come to mind are Elvis and Michael Jackson.
I have taken many animals that we have rescued over the years to the vet and this is how they always have done it. What I have stated is based on direct experience and what vets have told me.
I do not know how telling what vets normally do is going to scare anyone into anything. This is just another attack by you. Acting this way, you are just like a Big Pharma rep, trying to convince people to take things that are not safe. If someones liver fails from overuse of drugs, I do not want that on my head. If zappers were not safe to use or did not help many, I would not promote them.
On top of that, If I had worms in my stool, I could easily look and see whether they were round worms or flat worms and run up to the pet store or the farm store and buy the stuff off of the shelf. I would not be likely to take it.
The real question is 'is it smart to take these drugs, and especially in rapid succession?'