I know a lady who was wheelchair bound after a car accident earlier on in her life. Nevertheless, a guy fell in love with her and married her knowing that she couldn't have children. After years of marriage, she began to feel that he was missing the joy of having kids although he never openly said so. So she gave him permission to marry a second wife, which he did after a lot of hesitation. They are now one big family, two wives who are friends and a lot of kids who also love the first wife.
I see this as a success story of mercy and compassion. But there are other stories and other reasons for wanting a second wife. I say it works if the first wife permits it. And in a culture that accepts this practice and respects it, there is alot more benefit than harm done while preserving everyones' dignity.
But the best thing for all those who are really looking for the truth is to get to know a family that consists of more than one wife and find out for yourself if it really works.