I would say that it's an individual choice for each parent. In my professional experience a couple of sessions at a preschool a week combined with proper parenting the rest of the time can have noticeable benefits to a child. Most notably, they are given more freedom and independence at nursery and the opportunity to socialise with children from a variety of social backgrounds. I don't agree with parents who use it for their own benefit where they send them there five days a week so they can go shopping but I do realise that everyone is different.