I think the distributional consequences of polygamy are poorly understood. In a large sense, the policy redistributes from women to men. By constraining the "best" husbands from taking more than one wife, it reduces the implicit price of wives for less desirable husbands. Put differently, it constrains the choices of women about whom to marry, which can hardly make them better off. There is also, though, important implicit within-group redistribution as well.
One wonders if polygamy's good friend polyandry will make an appearance in China and India as the effects of selective abortion of women make themselves felt in the marriage market.