The biggest advantage in 'walking' marriages is that even as it gives max freedom to women and men, the children are automatically well taken care of. And the population gets regulated as well.
And the society as a whole is open about sexual relationships. In my reckoning I would consider such a society highly evolved. It has all the advantages of synergistic cooperative living.
If I understand well there is not much scope for bringing up children through polyamorous relationships; and even if that happens it's mostly by default.
Another discussion on 'walking' marriages