I really can't imagine loving a man and sharing a life with him but having him sharing his life with me and another (or more). I'm giving him my all and he's giving me his half (third, quarter)... And I know a lot of Muslim women hate the idea of it and struggle in trying to understand how they can learn to embrace this as part of Islam. Most don't have to worry about it, I guess, since polygamy is not that likely to happen in their marriage since it's not all that common and they'd hopefully have chosen or plan to choose someone with similar views to their's. But women who do think about it often find some conflict.
Anyway, to continue, I do think that polygamy has its uses in very specific situations. My first thought about it having its uses came when I saw something on TV quite some time ago about women in Afghanistan having lost their husbands. The ones profiled were having a very hard time making enough to take care of their children. They were really struggling (really impoverished), having lost the source of income of the family. It's in cases like this where I thought that marrying more than one woman would be good and almost necessary. In cases where lots of men have lost their lives and lots of women are left widows with little possibility of marrying.
I DO NOT think that a man marrying a woman for sexual reasons is doing anything good. In fact, I don't even think that it is permissible in such cases and I'd go as far as to say that in my mind, it's a sin.
In cases like I described above, the man marrying a second wife is taking on the responsibility of taking care of this woman (and her children, if she has some) and of course, will have the sexual benefits of a husband. And the wife might not be happy about it, but accepting to have this second wife really is an act of goodness.
I'm not very knowledgeable about the Qur'an at all, but I know this one verse talking about marrying more than one wife. I don't know if there are more that can shed a different light on the subject. And I do not speak Arabic, so I will just write 3 different translations in an attempt to get some of the interpretation out that comes with translating from original text.
YUSUFALI: If ye fear that ye shall not be able to deal justly with the orphans, Marry women of your choice, Two or three or four; but if ye fear that ye shall not be able to deal justly (with them), then only one, or (a captive) that your right hands possess, that will be more suitable, to prevent you from doing injustice.
PICKTHAL: And if ye fear that ye will not deal fairly by the orphans, marry of the women, who seem good to you, two or three or four; and if ye fear that ye cannot do justice (to so many) then one (only) or (the captives) that your right hands possess. Thus it is more likely that ye will not do injustice.
SHAKIR: And if you fear that you cannot act equitably towards orphans, then marry such women as seem good to you, two and three and four; but if you fear that you will not do justice (between them), then (marry) only one or what your right hands possess; this is more proper, that you may not deviate from the right course.
In this verse, at least in the translation, you can marry two, three and four, IF you fear that you will not deal fairly with the orphans. And what I read is that in Arabic, an orphan is a child who has lost his father (the mother could still be alive). It seems to fit with what I was thinking... It's to help children who have lost their fathers (and have no one to provide financially). The verse doesn't seem to say anything about whether or not the women the men marry have to be (or are) the mothers of these orphans, so it could also be to have extra women in the household if the man in question is taking care of many orphans. Women really are so valuable in educating children and raising them. What a wonderful man he is! I would fully support him having multiple wives in such a case. Not even a woman in that situation would like to be "co-wived", but she would be agreeing to allow in her marriage what is allowed by God. It would be an act of worship, in a way, for her to accept this agreement.
Note: I am basing this entry largely on my personal feelings about the issue. And I know I am. ;)