That's called a pyramid scheme, and it's not a good thing, because it's unsustainable.
However, given the overall birth rate in this country, the Duggars are just helping compensate for the many people who have less than 2 children.
I agree with you completely.
But from the point of view of someone who wants to know how fecundity "benefits society" - well those kinds of questioners generally are the sort of people who think Social Security and Medicare are great things.