I would agree that harvesting water from poisonous plants is sort of a last ditch alternative. There should be plenty of other options to choose from. I just can't imagine there being a whole lot of places where only toxic plants abound.
More to the point, I think the thread points out that if you are using the direct method you want to know what plant you are working with. That seems the more practical, useful point than to see how it could actually be done. Using my method you'd just about have to have a degree in MacGyverisms.
_________________________
The ultimate result of shielding men from the effects of folly is to fill the world with fools.
-- Herbert Spencer, English Philosopher (1820-1903)