Meadmaker, hello. I'll attempt to answer your question, because I think it's valid--very valid--and in fact goes to the heart of how society benefits from marriage in general.
Now, many people don't get married, or enter into relationships, to "help" society. But, as has been pointed out regarding heterosexual, or "traditional" marriage, there are benefits to society. One of those is increasing the odds of NOT living in poverty. Married people tend to have a higher standard of living than single parents. Why that is may be up for debate, but certainly a huge part of it is the potential for two incomes, having a sense of responsibility to more than just one's self, and, in terms of wanting what is best for a family situation, they tend to demand better schools, safer communities, cleaner parks, and the list goes on.
I just read your question, and the replies, so forgive me for this not being a well thought out response, please. I can only offer my own personal observations, and honestly, the current economic situation in and of itself, in my mind, only strengthens the positive aspects of having multiple partners (not talking emotions, here, just the non-emotional realities of relationships in general). In multiple-partnered relationships, households can actually function more successfully, in traditional ways, than "traditional" marriage, believe it or not. There can be multiple incomes, and still be someone home with children, or someone handling the home and personal business. There is increased buying power, for more than just essentials.
How that benefits you? Well, were it to become the norm? And the same rights as marriage given to poly families? I'd bet there's be less people having to utilize public assistance, government assisted health care, food stamps, etc. I'd venture to say we'd see, in the long run, reduced truancy--which is a very serious problem. Children really do benefit from having a parent there, and problem children really need closer supervision. Yet many single parents cannot do what some school districts are demanding they do...and are in danger of being considered criminals because of it. True, a single parent and child can live with another couple, and consider themselves "married"...but...wouldn't the situation be better for the child if the law would recognize the other adults as parents, too? We do for step-parents. And we call that good. Why can't a child have three parents in an in-tact family, and that be considered good as well?
Society benefits from marriage. You said it yourself--you believe you do benefit from your neighbor's marriage. Well, what are those benefits? And can you not see how some of those, if not all, would also exist in a poly situation? Perhaps moreso, given the potential for even greater household income? Poly families, after all, want the same things for their kids that you want for your own.