Well, first of all, the guy who posted this thread is in Greece. None of those species that you listed are found on his continent. Or on the same continent that pacs are from. The argument that those frogs are its 'natural' diet is kind of pointless in that case...

I do have a good anecdote about how toxins from frog species that are typically considered 'non-toxic' can harm reptiles/amphibians from different areas. I raise garter snakes, and they are known to be big amphibian eaters in the wild (NOT in captivity btw! I can't tell you how many people have had to deworm snakes after feeding w/c frogs). In fact, they're the only known animal with immunity to fire newt poison. But only garters living in the same locality as fire newts. Garters of the same species that didn't evolve alongside the newts don't have this immunity. That's an extreme example, but it's a clear one.

You say you've heard not to feed w/c prey hundreds of times; well, there's probably a reason for that. It's a risk. A scientific, quantifiable, risk. w/c frogs contain parasites. No, your frog won't automatically catch the parasite or disease after eating it, but it's a numbers game. Eventually something will go wrong.
I don't go around eating undercooked pork just because some of my ancestors were able to reproduce before dying of trichinosis.

Finally, like has already been said, there's no point in raising your own frogs as feeders. I don't have a problem with it ethically (they probably have a higher quality of life prior to feeding than pinkies/fish/ect. that are farmed and frozen as feeders). But why would you bother? You'll end up with way too many frogs to feed off, they don't freeze well, and you can't ethically release them into wild populations. Like Lija said, they're a last resort. Only time I've ever heard of it being needed is for w/c cornutas and for some failure-to-thrive baby snakes.