Unless the other parent is completely absent, you get that anyway. And even if they are (one of my parent was for a large part of my childhood) you usually get other family members, grandparents, ants and uncles, friends, teachers, etc.

While it is probably true that it is easier on the parent to have another person which is always there to help each other out, I doubt—all else being equal—this has any significant difference on the child.