Via RealClearBooks, here’s a sensible explanation of the Mormon practice of baptizing the dead by Samuel Brown, which he argues is part of a universalizing impulse as well as reflective of the faith’s nineteenth-century origins:
First, it is a solution to what some scholars call Christianity’s “scandal of particularity.” By this they mean that Christianity claims that salvation comes only through Christ. If that is true, though, what about those who had no conceivable way to hear of Christ, let alone to confess him? What justice is there in a Gospel that arbitrarily denies heaven to people merely by token of their place of birth? Joseph Smith and his Latter-day Saints answered emphatically, “None.” The Mormon solution to the scandal of particularity was not that Christ is unnecessary, but that Christ can be brought to everyone in the afterlife. While the notion offends many modern ears, the solution has a sort of ambitious coherence.
Second, baptism for the dead is a reflection of early Mormon ideas about the nature of family and human relationships. Though in the 20th century Mormons emphasized a more Victorian interpretation of these beliefs, early Mormon beliefs about family were stunningly universal. The family of heaven encompassed essentially every human being in early Mormon belief. Mormons understood baptism as the mechanism by which individuals were adopted into that vast family of heaven. On this view, baptism for the dead represents the hope that all of humanity will be united in the afterlife as one harmonious family. Mormons, rather than looking down at the damned with pious glee, are exploring every possible avenue to get the supposedly damned into heaven. That they employ the very physical rite of baptism to unite the human family reflects more than anything the assiduously literal and physical bent of Mormon thought.
Baptism is all hokum to me, whether it’s infant baptism, adult baptism, or baptism for the dead. I certainly understand that people are resentful of what they see as the imperious baptism of their dead relatives, especially relatives who weren’t of any branch of Christianity, but if you don’t share LDS beliefs, it shouldn’t really matter, should it? (And if you secretly believe postmortem Mormon baptism works, then maybe you should get yourself to the nearest stake and convert!)