Why do Mormons perform baptisms for the dead?

As a genealogist, I am in situations where the question in the blog title is frequently raised. Some of the people who ask this question are serious inquirers. Others are antagonistic to the principle altogether. It is important to realize that as members of The Church of Jesus Christ of Latter-day Saints we have a valid, scripturally supported basis for our beliefs concerning this important ordinance. Quoting from the Frequently Asked Questions section of Mormon.org:

Jesus Himself, though without sin, was baptized to fulfill all righteousness and to show the way for all mankind (see Matthew 3:13-17; 2 Nephi 31:5-12). Thus, baptism is essential for salvation in the kingdom of God. We learn in the New Testament that baptisms for the dead were done during the Apostle Paul’s time (see 1 Corinthians 15:29). This practice has been restored with the establishment of The Church of Jesus Christ of Latter-day Saints. The Prophet Joseph Smith first taught about the ordinance of baptism for the dead during a funeral sermon in August 1840. He read much of 1 Corinthians 15, including verse 29, and announced that the Lord would permit Church members to be baptized in behalf of their friends and relatives who had departed this life. He told them “the plan of salvation was calculated to save all who were willing to obey the requirements of the law of God” (Journal History of the Church, 15 Aug. 1840).

Section 128 of the Doctrine and Covenants is a letter from Joseph Smith containing instructions about the principle of baptism for the dead. The basis for the practice is explicitly set forth in Verse 8:

8 Now, the nature of this ordinance consists in the power of the priesthood, by the revelation of Jesus Christ, wherein it is granted that whatsoever you bind on earth shall be bound in heaven, and whatsoever you loose on earth shall be loosed in heaven. Or, in other words, taking a different view of the translation, whatsoever you record on earth shall be recorded in heaven, and whatsoever you do not record on earth shall not be recorded in heaven; for out of the books shall your dead be judged, according to their own works, whether they themselves have attended to the ordinances in their own propria persona, or by the means of their own agents, according to the ordinance which God has prepared for their salvation from before the foundation of the world, according to the records which they have kept concerning their dead.

Those who are members of the Church have more reasons to seek after our dead and do the work of genealogy or family history than those who do so out of an interest or hobby. As Joseph Smith said,

The greatest responsibility in this world that God has laid upon us is to seek after our dead. The apostle says, ‘They without us cannot be made perfect’; for it is necessary that the sealing power should be in our hands to seal our children and our dead for the fulness of the dispensation of times-a dispensation to meet the promises made by Jesus Christ before the foundation of the world for the salvation of man.
( History of the Church, 6:313. )

 This responsibility is not discharged by planning to do your genealogy sometime in the future or when you retire. I did not retire from work, I retired to work. I merely changed the emphasis of my full-time profession as an attorney, to take on another full-time profession, that of genealogist and teacher of genealogy. I see too many of my contemporaries, who are now at or past the retirement age, pursuing an "active adult lifestyle" focused on themselves and their desires and needs and not on taking their, now retired time, to do what they put off for so long. I haven't ever seen a time limit placed on our duty to seek after our dead or that we only need to do so when it is convenient or not interfering with our "active lifestyle."

I suggest that those who question the doctrines of the Church and claim that Mormons are destroying the cultural heritage by baptizing their ancestors, would deny those same ancestors, who are active, individual spirit children of our Father in Heaven, the same rights to determine their own beliefs and actions as those same people would so defend for themselves.

If you do not believe in an afterlife, then baptism for the dead would seem unnecessary. If you do believe in an afterlife, what gives you the right to determine how your loved ones who have passed on want to believe or what they want to do in that afterlife? If you are a believer, and also a member of the Church, why would you deny those same ancestors the right to be baptized if they wished to be? How is your failure to do your own genealogy and seek after your own dead any less of an issue than those outside the Church who would deny them the opportunity through stopping the practice of baptisms for the dead?