That is the reason the 2n/0 model is still sensible if you are raising more than 2 kids and/or consider raising the kids yourself (mainly by the /0 partner) has a higher value (e.g. by unavailable of comparable education) than the income you could earn in the time the children by school/kindergarten. This may depend on your country/views.
Dalrock writes an interesting article related to Dr. Helen Smith's book the Marriage Strike. I really have to bump it up on my too rapidly growing reading list. (^_^)