In my not-so-humble opinion, the doctrines found in section 137 are among the most important in the Gospel. In fact, just last night, I was talking to a roommate of mine about this idea. One of the axioms of Christian religion is that God is perfect, i.e. fair and just and merciful. All of our beliefs must measure up to that standard, or need to be very closely examined. God's fairness is an incredibly base belief of Christianity.
However, many, or even most, mainstream Christian belief systems present beliefs that deny this basic principle. For instance, Calvanism, though not a large church in America, teaches that some are predestined to be saved or damned. How is this fair? Was it based on something we did or did not do in the pre-existence, which they deny the existence of in any case? No, it's just essentially random, with nothing we can do about it. Many other religions, in official doctrine, such as the Catholic church, Anglican Church, and many other protestant denominations, claim that without baptism, one will be damned. Ok, fine, but what about all those that never had a chance to be baptized, especially if not into your church? Well... According to official doctrine, they are damned too. That just doesn't seem fair to me. Is one person cast off simply because he was born in the wrong place? Am I to be lost for something that was no one's choice? This a conflict in these religions. How to reconcile?
However, most, or at least many, members of these churches don't believe the official dogma of their religions. They will instead say that there are many paths to heaven. It seems they do this in response to the feeling they have that God must be fair, that if he wasn't, he wouldn't be God. And in the absence of doctrine teaching about a chance after this life, and vicarious baptism, they must accept this. There is a spectrum of this attitude, from evangelical "just believe in Christ," to an even more open "all religions lead to God." But all try to reconcile God's fairness with a need for judgement. But few, if any, other than the LDS church, has a doctrinal stance that allows the church to be both authoritative and have a fair God.
The answer is that all will have a chance to repent, in this life or the next. Black, white, red, yellow or other, everyone will be given this chance. And baptism is still vital. You must be baptized. But, at the same time, if you never had a chance, you can be baptized after this life, vicariously. God can be both fair, and demand justice. Amazing, isn't it? The Gordian Knot has been unraveled.