One of the biggest issues in our missionary efforts among other Christians is that it is almost impossible for many who have been indoctrinated with other interpretations to look at the Bible and parse the big picture - to break away from their former teachings and see what the Bible (especially the Gospels) actually teaches. There is a HUGE difference between believing in the Bible and believing what the Bible actually says, and too few members realize that most of the Mormon Church's most unique teachings and doctrines do not come from the Book of Mormon; rather, most of them come from the Bible.
I know this is a broad generality, but most of Protestantism doctrinally is Pauline- and creed-based, while Mormonism is much more Gospels-based. That is a clear, bright-line difference in many, if not most, cases. Ironically, the best way to break that Pauline-/creed-based paradigm for many people is to gain a testimony of the Book of Mormon, join the Church, receive the gift of the Holy Ghost and then start seeing and reading the Bible with a different light. I have seen it over and over and over again, and it fits perfectly with Mormon 7, especially verses 8-9.
If someone is a Bible believer and a seeker, and if that person is open to a thoughtful discussion of the Gospels as the foundation of Christian theology (rather than the creeds and Paul's epistles), then it is not hard for someone who knows the Bible well to help them begin to see the Gospel in a new light and understand the need for a "Restoration". However, they still need to read and accept the Book of Mormon in order to gain a testimony of that restoration through Joseph Smith.
Most young missionaries just aren't doctrinally mature enough to tackle both of those approaches, so the lessons are structured to focus on the second (gain a testimony of the Book of Mormon and then start to understand the Bible correctly once you are a member of the Church). Therefore, the responsibility to "nurture new members in the good word of God" falls on the members in most cases - not the missionaries. It also highlights the need for members of the Church to know the Bible better than we generally do.