There are immediate benefits to having children. They make you humble. They push you to be more than you ever thought you could be, because they challenge you. They allow you to do childish things again, without feeling stupid about doing them. (When is the last time you finger painted, played with play dough or built a fort or snow man?) Having children is in some ways a refresher course in the greatest simple joys in life.
Sure, I don't have a problem with people having children because of immediate benefits. That is when one should find whatever benefits you expect.
My objection is to the idea that people should have children so that we can turn them into de facto slaves when they become adults. There are many people on FR that view children as little more than future adults to suck taxes from or to serve them in their dotage. It is a pretty sick view, but a lot of people have it.