I don't know all the details - that just came up in my research when I was writing about polyandry a couple years ago. I'm assuming that if one husband dies in war, there is at least another one to continue fathering children with the wife, as well as helping to take care of the existing family. One man with many wives could father a lot of children, but he would not only need to be wealthy enough to support all of them, but it leaves younger/poorer men with no wife, which can create issues. We see this with Mormon polygamy where various pretexts are cooked up to run younger men out of the community. It's a bit destabilizing.