People who view it from their moral high-horse ought to realize that they have some pretty thick cultural goggles in their way. Just because it’s seen as immoral and weird in the US doesn’t mean that it is so.
A clear majority of world cultures either allow or actively support polygamous marriage; it’s not just Muslims and Mormons. Polyandry is rare, so this most often takes the form of polygyny. Even within those cultures, having more than one wife is usually fairly rare and is a sign that the husband is capable of supporting them. All cultures have rules that govern marriage, and that holds true for those that practice polygamy. For example, the Koran sets rules for how the wives are to be treated, particularly materially.
Some of the reasons for polygamy are economic and practical. In one of the few places that still practices polyandry, northern Nepal, both trade and herding are important for economic success. If the man is away trading, he can’t be herding or farming; if he’s herding or farming, he can’t travel for trade. Usually, two brothers will get together and choose a woman to marry. Sometimes, they will take on another wife too, creating a group marriage. The main reason for this, according to them, is to prevent their very limited land from being split up among too many heirs. In other words, it’s economic. It also has the effect of limiting population to available resources.
In many cultures women bring a dowry, and in all cultures marriage solidifies relationships. That friend of a friend who helps you out finding a job would have more of a vested interest if he were a relative by marriage to one of your wives. Multiple wives share multiple duties, taking the roles of maids, nannies, and other household services which the well-off among us hire non-family members to do in our society. How many people in the West have kids who are raised mostly by strangers? How many people can afford to pay for all those services mentioned above? In agricultural societies, more children mean more hands available to do the work. Having a large family might be the difference between having a comfortable dotage and having your few children abandon you and the uncomfortable borderline poverty you raised them in by the time you’re old.
In the US and Europe, multiple marriage or some form of it often exists anyway. When I was a kid, many of my friends were envious that my parents hadn’t been divorced. A quick glance at the divorce statistics shows that many, many children are raised by more than one set of parents. In some cultures, mistresses are almost a requirement of success. (A French acquaintance joked about the Clinton scandal, “if someone as powerful as the President did not have a mistress, we would question his sexuality.”) Even in the puritanical US of A, a divorce due to focussing on a career, and a subsequent marriage to a second (usually younger) wife is seen as one of the penalties and perqs of being a successful businessman.
Personally, from all the difficulties I’ve seen in ethnologies about and literature from many different cultures, I wouldn’t want a polygamous marriage. Trying to balance everything is hard work. You’ve got to be more of a go-getter than I am to keep two or more women happy, even in cultures that grew up with those ideals. Handling the lower demands of a wife and a mistress would probably be more trouble than it’s worth too.
So, I’ve got no problem with other people doing it — certainly no moral objections —and I see the advantages inherent in the system, but I don’t think I’d want two wives. I don’t even want to picture what a headache the four maximum Islam limits a man to would be. (I mean, every time you bought a thoughtful little gift for one, you’d have to get the equivalent for the rest. Fegh, no thanks, I’m not thoughtful enough, successful enough, or imaginative enough for that.) I may joke with my fiancee that she can pick my second wife to help her when I’m rich and powerful, but the social realities make it clear to me that I’d rather it stayed a joke.