Studies have shown that people who grow up without siblings tend to be more independent than those who do. They practically grow up having to learn things on their own.
The first day of school? There won't be any big sister to show them around. The first fight? Big brother Jimmy
won't be there to defend them. First crush? No advice on makeup from their sisters. First car crash? Gotta settle it themselves. So in a way they generally turn out tougher than other kids.
Not to mention that this independence is one way parents can rest assured that their child won't feel like following the lemmings over the cliff later on in life.