If Mormons had to believe every opinion of every leader the church has ever had, let's face it -- Mormons would believe some crazy and even contradictory things. Critics of the church often try to use that against us -- taking fringe ideas as if they were mainline doctrine. But this leads to the issue: what is Mormon doctrine and what is not? And why would a prominent leader teach something that isn't true or that isn't doctrine?
Here's my view: In the church there are two different types of teachings: doctrines and interpretations of doctrine. The doctrine is always true, but an interpretation of doctrine doesn't have to be. I won't get into what constitutes Mormon doctrine (that will be for another time) but I do what to point out a few things with interpretations of doctrine.
An interpretation of doctrine is exactly what it sounds like -- an interpretation. There is no requirement or even always a need for it to be true. Interpretations are meant to be specific and practical applications of doctrine. These are the "how do I apply this to my life" things. They also come because at the time they are the best or easiest way for us to understand things. A recent example is changing one word in the introduction of the Book of Mormon from "principal" to "among." Many years ago it made the most sense if all the Native Americans were of Lamanite decent. This was never a fundamental concept and knowing for sure of its veracity had no effect on the salvation of any individual. However, recent DNA evidence suggests that there are Native Americans who do not have any Jewish blood in them. Because the idea was only an interpretation it was easily changed from all Native Americans to just some of them. Even still there is no need for any of the current Native Americans to have Jewish DNA.