If three or four people want to raise a family together,* the kids would be probably be even better off than a double parenting. There's probably a point of diminishing returns--I wouldn't want to be raised by a hundred parents. But I doubt two is the optimal number.

From the parents perspective, most people aren't interested in a poly relationship. And most poly relationships aren't stable for the 20 years it takes to create an adult. So while a poly relationship might be better for the kids, it isn't always feasible.

Now I wouldn't begrudge anyone, even a single person, who wanted to reproduce and had the money and time to do so. Even if a child is raised in a sub-optimal family structure, she's still better off than if her parent(s) had decided not to reproduce at all. But we can value the contributions of sub-optimal family structures without pretending that they are optimal.

*"Together" is the operative word here. Two couples living in separate houses trading kids every other weekend isn't going to cut it.