I'm not going to say I completely agree with the decision to have that many children.
I will say:
There was a time that it was GOOD to have children that learned useful skills and were taught how to be a part of a family. Children now a days don't learn how to do basic, ordinary skills that can serve them a lifetime.
Gardening, canning, knitting, crocheting, taking care of livestock, learning to build homes from ground zero...these things are rarely taught to children. Instead, they are taught to watch t.v., play video games, skateboard, 'hang out' aimlessly with their friends after school doing nothing useful. We're constantly "finding things" for our kids to do that really teach them valueless lessons. After school activities that focus primarily on academics, athletics or just keeping them out of trouble, take up so much of their time that they have no time to learn life-skill lessons that could serve them much better than academics/athletics alone.
I know More people in my age group than not that have little idea how to care for a garden or even cook from scratch.
I give these people credit for taking their responsibility as parents serious enough to make sure that their children are home and being taught valuable life lessons and skills that will stay with them a lifetime, as opposed to the people who honestly would rather shuck the responsibility of raising their kids onto the public school system and government.
I hardly would call these children slaves, seeing as they have food on their table, a nice home to live in and parents that truly care about them. Perhaps I don't agree with their religious beliefs that led them to this decision, but teaching your children to be an active member of the family and teaching them responsibility is hardly the grounds for slavery.