Note that, in order to disprove Mormonism, we need not know for certain which view of the passage is correct, so long as we know a possibility that fits the passage and other passages.Thus, it is less important to actually understand the meaning of the passage than it is to prove that there might be some other explanations. I understand the desire to see things this way.
When a person joins the LDS church, a person must be baptized, whether or not they were baptized into some other church before, because in a sense, the basis of the LDS church is that the world suffered a great enough loss of gospel knowledge and authority through the absence of prophets and apostles during the past two millennia that God saw fit to call a prophet and restore it all. To become a member of the LDSLDS doctrine holds that God will only uphold baptisms performed under His authority (i.e. through the LDS church). If you assume that God really did call Joseph Smith as a prophet, this point of view actually makes sense. But if you begin with the assumption that God can't or won't call prophets anymore (a theory for which I have yet to find any solid basis, but one which nevertheless has been a basic tenet of most Jewish, Christian, and Muslim sects for well over a thousand years), then it's really easy to see this as an affront. It basically says to the Christian world, "You're wrong." Or at least, "You're wrong about some of these things." And while the Christian world can't seem to agree on any single set of doctrines, most individual Christians believe that such differences of opinion are acceptable in order to maintain unity in what they see as "The Church," or the collective body of all people who believe in Christ, regardless of differences in doctrine and opinion. But to have one church arise, claiming not to have branched off from any of the other churches, but to have been founded by Jesus Christ himself, and saying that no baptism performed outside its purview will be recognized by God at the last day.... well, it's not easy to take that sort of thing lightly. I mean, when you've accepted Jesus into your life and seen the great change that the Holy Spirit has wrought upon you, and then somebody says there's more to it than that, it's easy to perceive it as an attack on some very important parts of your life--an attack on your faith itself. And for leaders of those churches, who have consecrated so much of their time, faith, and energy--to say that their church is somehow lesser than some other church is a hard thing indeed. It is only natural for them, quid-pro-quo, to refuse to recognize LDS baptisms, as the LDS church does not recognize theirs.
And so it it not surprising to me when leaders of other churches don't wish to cooperate with the LDS church. It is a hard thing to overcome the perceived slap-in-the-face and seek to do what is beneficial to everyone. Certainly, members of the LDS church have often had difficulty turning the other cheek in all circumstances. Many of the early members fought back against the mobs that sought to turn them out of their homes, against the direction of the church's leaders. But that's not what Christianity is about, is it? Christianity is about putting every ounce of pride, envy, hate, and malice that we have "upon the altar," and allowing it to be consumed. I do not claim to be perfect in this sense; only one man ever was. But it's the thing we strive for. In becoming Christian, we say to God, "I will seek above all else to follow the example and commandments given by Him whose name I bear." And so it pains me to see Christians, and especially leaders of Christian churches, acting out of any less-than-righteous motives. And that, unfortunately, is what I see in this decision on the part of the Catholic Church.
If the Pope felt that it harmed the souls of those for whom baptisms were performed, then I would not feel this way. In that case, he would be acting to serve the best interests of those peoples' souls, most of whom undoubtedly lived good lives according to the Christian principles taught by the Catholic church in their day. If there were any chance that this were the reason for his decision, I would assume that he was acting out of pure altruism. I tend to assume that people are acting out of the best intentions that I can possibly imagine them having. And I know, ultimately, it is not my place to judge the man. This isn't between him and me--it's between him and God. Nevertheless, I am taking advantage of the right I have to voice my opinion. And my opinion is that this new policy of the Catholic church is motivated by pride, and is enacted without due regard to the good of everyone involved. "Mormons" seek to perform genealogical research out of a sincere desire to bless those who do not have the power to help themselves. The Pope can believe that they're wasting their time. But there is no grounds for him to believe that it could in any way harm the people for whom the ordinance is being performed. Either you believe that the LDS Church has authority from God or you don't. If you don't recognize baptisms performed by the LDS Church, then you don't believe that they have any effect, right? So in what way does it harm anybody when they perform them?
I know I've met Catholics who thought that by performing baptisms for the dead, we were somehow forcing them into church membership without their say-so. Those who felt this way were never willing to take the time to understand any differently. Had they been willing, I would have explained that by our own doctrine, we are simply giving these people the opportunity to accept a baptism which was performed on their part. Did the Savior die only for those who wanted to be Christian? No. He gave himself as a sacrifice for all mankind. But will all mankind have a heavenly existence forced upon them? No. Clearly those who reject holiness and choose a life of sin will not have their place in the Kingdom of Heaven. If you believe that Christ's atonement was necessary for our salvation, then you believe that He opened the door to those who would be saved. Jesus taught that baptism is a requirement for entry into His Father's Kingdom, but does anybody believe that if a person is baptized against their will, they will somehow be forced into heaven against their will? A baptism-for-the-dead, even if you believe in such things, can only have force if the baptised recognize it. If you don't believe in such things, then it surely can do no harm. So why are people so opposed to it?
But perhaps I'm being too hard on them. Perhaps they see themselves as taking the opportunity to prevent us from damning our own souls. Would a good Christian farmer sell his pagan neighbor a chicken, if he knew it could very well be used in some kind of forbidden ritual? Would a patriotic American ever sell a flag to someone who might be planning to burn it? As any decent person would refuse to allow a suicidal friend to borrow a weapon, perhaps the Pope sees it as his moral duty to prevent his church from permitting this wicked practice, as long as they have any way to prevent it. Maybe he believes that, by removing our capacity to sin, he is somehow bettering our eternal situation. Such a belief could easily become the subject of an enormous theological debate: if you deny someone the agency to choose what they believe to be right, but which is actually wrong in the eyes of God, and the decision has no bearing or effect on anybody else, living or dead, will God be more tolerant, in the end, of their desire to have done it? Maybe that's his reasoning.
I doubt it though. I'm pretty sure it was intended (and will be interpreted) to be a measure taken by the Catholic church to place a stumbling block in the way of Latter-day Saints. The two churches have shown remarkable cooperation in recent humanitarian aid efforts, and here we have the Pope saying, "It's fine for us to cooperate when we're being charitable towards others, but I won't allow my church to share information with you that will help in your genealogical efforts." Nevermind that the information that the Church gleans from such efforts is made available to members of all faiths, including the Catholic who wrote the first article I linked to in this posting. Mormons are not the only ones who have had their hearts turned to their fathers, and I think the consequences of this policy may be much more far-reaching than the Pope thought or intended.
That said, it is his decision to make. I recognize his authority to withhold such information, regardless of whether I like it. And may God always grant him the inspiration he needs as a man charged with the spiritual leadership of so many people around the world.