The Mormon Times has a short write-up on a presentation given by Terryl Givens at UVU (that's Utah Valley University) on the preexistence in Western thought, the topic of his soon-to-be-released book. While the doctrine certainly has its appeal, it remains a heresy to orthodox Christians. Here, quoting from the article, is one purported theological advantage of accepting the doctrine.
In most historical writings that reference a pre-mortal existence, Givens said the pre-existence served as a solution to a theological problem -- usually that of the argument against the notion that men have free will.
Quoting John Taggart, Givens said, "'If God created our souls, he could have prevented all sin by creating us with better features and more favorable surroundings.'" Givens went on to say "(Taggart's) argument led him to conclude that ... a human spirit rooted in an eternal pre-existence solved this dilemma."
So defenders of the preexistence suggest that holding God to be the Creator of human souls lays some or all of the responsibility for sin at God's feet. Instead, they posit an eternal past for human spirits or souls, mirroring an eternal future. If spirits are uncreated, then God is absolved of the responsibility for having created our flawed nature and corrupt will, which give rise to the world of sin we know so well.
But I'm not sure positing a preexistence or eternal and uncreated souls really delivers on this promise to absolve God. Between a host of preexistent heavenly spirits and a world of bodily enfleshed spirits lies the problematic process of matching spirits to bodies. Problematic, of course, because not all bodies are the same. They come with radically different initial endowments (as an economist would say), including a genetic inheritance that contributes an undetermined but substantial measure of one's earthly disposition and predilections. If God assigns spirits to bodies, the same degree of responsibility seems to follow as if he created spirits using a just-in-time process. "If you had matched me up with a healthy mind and body and with decent parents, I wouldn't have been so evil" seems to be an objection that raises the same or similar issues to just-in-time creation. And it would be hard to avoid the objection by suggesting the matching process occurs independent of God's control.
Originally posted with comments at Beliefnet.