Turns out I still have those missionary training materials. I won't say when the copyright date is, I'll just say that the materials came with cassette tapes.
There was a section that talked about sharing experiences with investigators, the relevant bits:
There was no reason given for the counsel, we were left to come up with our own reasons. I remember one was that it inadvertently sent the signal that you could sin, repent, and turn out awesome... counter intuitive right, isn't that the message of the gospel? But the idea was that people might rationalize and decide to enjoy a little sin in the present, repent later, and be as awesome as missionaries are. I'm not saying it holds to reason, it was an apologetic that a group of kids came up with to explain the "whys" that weren't included in the training materials.When you are sharing experiences, be simple, clear, and direct. Share only those parts of the experience that relate to the subject and will uplift the investigator. Use words that the investigator will understand. Do not share past transgressions, even if you are trying to help a person who is having problems that you have had.
The suggestion is also in the Preach My Gospel training materials (which I've come to understand the PMG materials will be replaced soon?):
If I were to guess at a reason for the suggestion I might say that they feel discussing sin might drive the spirit away? Or put a dent in the veneer of perfection that the culture likes to create for itself.Lesson 3: The Gospel of Jesus Christ wrote: Sin
The notion of “sin” means different things in different cultures. In some cultures it is closely associated with the concept of committing crime. In others it applies only if one is caught doing something wrong and thus brings shame to a family or community. Clarify that sin is disobedience to God’s commandments and results in becoming separated from God. God knows all that we do and think, and we displease Him when we sin. Do not discuss your past transgressions. Discourage or ask investigators not to discuss their transgressions.