The double negative in your question is a bit confusing. I think you're asking why it's a bad idea to avoid birth control.
Children take a LOT of time and energy, not to mention the financial needs. (I think the $250K per child figure is way too high, but still, it does take money). Most parents have limited time, energy, and income, so they need to adjust the size of their family accordingly. In addition, constant childbearing is hard on a woman physically. Pregnancy and childbirth do have inherent risks and can cause injury to the mother, even in our day of modern medicine. Some women, especially those who are introverts (which doesn't mean shy; it means they need significant amounts of time to themselves to feel mentally and emotionally well) or who have some form of ADD, can't function well in a house full of children.
If you're talking to someone who believes in avoiding birth control for religious reasons, none of this might matter. It's very hard to talk someone out of a deeply held belief. It doesn't always go well for those families, though. Sometimes they create a life of hardship for themselves.