I recall a Muslim poster once defending polygamy as essentially a concession to human nature, that men are naturally polygamous, and that in non-Muslim countries men take mistresses anyway, but those women don’t have the same rights and protections a second wife would.
I can see some pragmatic arguments for polygamy, such as, since most women who are divorced live in poverty, as do their children, maybe if polygamy was an option there would be less legal divorce, men tired of their first wives wouldn’t divorce them but just take an additional wife, and some first wives who can’t support themselves financially, might find that the lesser evil compared to divorce.
Also, the FLDS and other Mormon splinter groups that practice polygamy, will actually take advantage of the illegality of polygamy, since wives who are “spritually” but not legally married, will qualify for welfare benefits as single mothers, that they would not qualify for as married women.
While I do realize many TV documentaries these days are far from accurate, I have watched episodes of A+E’s “Escaping Polygamy” which depict the “spiritual wives” as little more than glorified concubines, one of them was living in a shack that was so dilapidated her daughter (who had left the group) was trying to get it condemned as unfit for human habitation as a way to persuade the mother to leave her “husband”.
And much as many argue for the legalization of prostitution, arguing that this will provide the “sex workers” with more legal protections, allow them to unionize, etc., I can see an argument that people who believe in polygamy are going to do it anyway, and legalizing polygamy will provide more rights and protections for the plural wives than they have now.