The best way to understand the WHY behind human behavior isn't to listen to what people SAY about "why". People tend to create post-hoc explanations that justify behavior, and those explanations may or may not accurately portray the real "why". You must look at the results of the behavior, particularly a repetitive behavior. The results tell you the "why".
The results of polygyny are predictable. In fact, there's only one predictable result: certain men, normally alpha males who are powerful leaders in the community, end up with more wives and offspring.
Polygyny, as practiced in the early LDS church, was not about taking care of widows and orphans. For one thing, widows and orphans could have been taken care of without a marriage taking place. For another thing, the most powerful men in LDS history weren't marrying widows and orphans who had no one else to take care of them. They were marrying the more desirable women who often had other candidates for marriage. They were marrying women who already had husbands. The way polygyny was practiced in early LDS Utah had the result of having a shortage of potential wives. Younger men served missions, and part of the purpose of their mission seemed to be to find a wife, and bring home other eligible females.
As it was on earth, it will be in heaven, if Mormonism is true. That means that the most powerful alpha males, who can be recognized by the fact that they have high callings in the church, usually as general authorities, will be rewarded by having multiple wives and more offspring - even if the cost is that less worthy - or beta males - will have to do with no wife at all.
Joseph Smith, in discussing polygamy with Benjamin Johnson, told him to listen carefully to his sermon that night, which would contain a message that only he would understand. In that sermon, Joseph Smith talked about the parable of the talents.
http://www.boap.org/LDS/Parallel/1843/2Apr43.html#N_7_
What is the meaning of the Scriptures, he that is faithful over a few things shall be made ruler over many, and he that is faithful over many, shall be made ruler over many more? What is the meaning of the parable of the ten talents? Also the conversation with Nicodemus, except a man be born of water and of the Spirit? Were questions put to one which I shall not answer at present.
The reference was apparently to plural marriage, according to Benjamin F. Johnson (note 2) who claimed a conversation prior to the meeting involved Joseph explaining the doctrine and saying he would preach something only Johnson would understand (see Johnson's autobiography).
Many sources, including Kathryn Daynes, whom LDS trust as a source, assert that Brigham Young went on to explain the connection between the parable of the talents and polygamy.
http://books.google.com/books?id=8RuWiM ... ts&f=false
Young said monogamists were the slothful servants who would have even their ONE wife taken from them, and given to faithful polygamists.
I'm not referring to these things because they're official doctrine. I'm referring to them to bolster my argument that polygamy was not really about taking care of excess females. It was about rewarding the more righteous and deserving alpha males with increased wives and offspring. And females would be better off with that alpha male than with being the one wife of a faithful monogamist.
This is one part of early Mormonism that has become somewhat disconnected to modern Mormonism: the idea of a royal bloodline within the faithful. It comes from the Book of Abraham scripture that asserted that some spirits were more faithful and righteous in the pre-existence, and would be sent as leaders of the LDS church in this life. They're the ones on a fast track to exaltation and glory. They're the ones that deserve more wives and more offspring.
I think it's sad, actually, to see faithful LDS men defending the practice of polygamy, when, if all these things were actually true, they will likely be victims of the practice themselves.