For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Ted Beaton
Mormon's also believe that everyone gets a second chance after they die to "hear" the gospel and to accept it. So they do the baptism of the dead to "get that out of the way for them." They defend this belief by misinterpreting 1 Peter 4:5-6 By taking the verse out of context and not realizing or ignoring the fact that the Bible uses the terms "quickened" (being brought to life spiritually) and "the dead" as being spiritually dead as opposed to physically dead. See Ephesians 2:1 and 2:5. One other note for Mormons, if you discuss the Bible with them, it saves a lot of quibbling if you use the KJV only. They only recognize that version of the Bible.
Michael Smith
My problem with the Mormon's work with genealogies is it's ripe for fraud. People I know don't rely on Mormon genealogical work due to the fear that much of the generational work is made up. My family reported that the farther back you search the more you find "ancestors" that have Biblical names. Even though our lineage is Swedish. Be suspicious of any doctrine that becomes more prominent than a relationship with Jesus Christ. It will by its nature and by human nature become corrupt as it begins to become difficult to substantiate. Also, most people don't have an alternative verification system. So human nature dictates that the information is not going to be trustworthy, nor is it related to Salvation. So it's motivation is a lie to begin with.