Anytime where you have an institution of multiple wives but not multiple husbands is a mathematical disaster waiting to happen, IMO. I've got no problem with the idea of one person having multiple intimate relations, but it's not something I could handle on my own, i.e. there is no way I could be a good boyfriend or husband to more than one woman at a time.
But I am very intrigued by the idea of "line marriage" from The Moon is a Harsh Mistress. In that system, it was a union of adults, and every few years a new person was added to the marraige, and it alternated between adding additional males and females. I would work out that the spouse(s) people were closest to were their relative peers (i.e. the people who were the same age and joind near the same time) whereas the older spouses were more like really intimate siblings.
The real pros of this system, as I see it, is you get the intimacy of monagamy, the variety of polygamy coupled with the wisdom, experience and guidance of older generations, all wrapped up into the same package.