This seems more a result of religious fanaticism. They state they will continue to have children as long as God wills, but it is THEIR actions that keep the mother in a state of near perpetual pregnancy. Barring any real kind of birth control, abstinence is 100% effective, as many evangelicals will say. They don't seem to be using that. Even the rythm method crowd have better results than these guys. I'm pretty religious myself so I can't knock them for their devotion, but even I think they take that whole "be fruitful and multiply" thing a little TOO literally. It boarders on a psychological condition here.
The mother has been quoted as saying that she isn't happy unless she has a newborn to take care of or is pregnant. Past a certain point, you can't actually raise your kids, you pawn them off on the eldest, who are often still children themselves. Even if you could financially support them all without any outside assistance whatsoever, I do not think that it is fair to the children emotionally and developmentally speaking. There simply isn't enough time in the day to really focus on that emotional bond. Heck, my own mother was 9/10 children, so I can see the results of the compromises that must inevitably be made when trying to bring up such a large family. I can't count all the cousins I have at this point...hehe..
Here's a real gem from the TWOP forums that really puts the whole "quiverful" brood into perspective:
When a quiver walks in
Looking real chaste
With that hoop skirt in ya face
You get sprung