I think poligamy can be seen from different points of view. From a darwinian point it is a good thing for a man that want to increase the possibility to transfer his DNA to more descendents as possible, for a woman it looks less interesting, bur if there are few men around she has the same darwinian aim, so she can share the best man available with other women. In this time I don't think we have these things in mind, at least consciously, so poligamy can have only religious or traditional origins if we born in countries where it's practiced. The disavantages can be psycologic, if you are not used, I think it's quite difficult to share your beloved, with others. Sometime it's difficult to marry one person, think about marring four.... or love four in the same way.... Depend on whether you accept it or not and where you lived. But what about polyandry, this is still more complicate for me ...