The Book of Mormon contains the fullness of the gospel of Jesus Christ. Everything needed for a person to find out how to return to the presence of God while in the flesh can be found in the Book. One way to look at the Book of Mormon is to see it as a compilation of accounts about various people that successfully navigated the path back to God's presence.
"The scriptures are accounts written about and by people who have received the second comforter... They tell us what to do... We should be eager to look carefully at what they tell us about how they got there." (The Second Comforter Conversing With the Lord Through the Veil Pg 27)
Of course, among the many scriptural accounts of those that successfully completed the course back to His presence, is the record of some serious failures. The records of the failures in the Book of Mormon were left there for a good and wise purpose in the Lord. We should pay particular attention to the accounts of those that once were righteous and prosperous in the Lord, then ultimately failed and dwindled in unbelief. That cycle repeats itself in the record noticeably.
The contrasts built into the Book of Mormon narrative are fascinating and instructive. The contrasts are placed there in order for us to see and understand something about ourselves. If we pay attention, and prayerfully consider the accounts of the likes of Enos, and Nephi, for example, we may be taught in great specific detail, how to enter, traverse, and complete the path back to the presence of God during our mortal lives. This is the fullness of the gospel of Jesus Christ and the greatest purpose of the Book of Mormon. On the other hand, however, we should make very careful note of the failure of what was once a Zion society at the end of the Nephite record. If, with the help of the spirit, the records of Enos and Nephi can show us in surprising detail the practical application of the fullness of the gospel of Jesus Christ and entering into His presence; then the Nephite Zion's dwindling in unbelief shows in equally striking detail how people move quickly out of God's favor; even when they were once a “chosen” and blessed people of God.
It was by using this glaring failure of the Nephites that the Lord warned the latter day church. "...but beware of pride, lest ye become as the Nephites of old." (D&C 38: 39) That is just the point, the failures recorded in the Book of Mormon are in fact warnings to the latter day church and people, "lest we become as the Nephites of old." That warning by the Lord was given in December 1830. How would we answer the question now 180 years later, have we become "as the Nephites of old?" Or, because of our “chosen” status, and the glorious destiny that awaits the latter day church, have we successfully avoided the calamity that befell the Nephites of old? Remember, they once had Christ dwell personally among them. In contrast, the latter day church failed in every attempt to establish Zion. It is in fact possible to begin at a lofty "chosen" status and then subsequently lose it all. Latter day saints don't generally believe it is possible for them or the church to fail. Latter day saints tend to disregard the warnings in the Book of Mormon because they believe the divine destiny of the restored church precludes failure.
The recorded failures in the Book of Mormon are not placed in the record to point out to us the weakness in others. They are instead, meant to show us where we need to change, repent, and be different. The warnings interspersed throughout the Book of Mormon record are in fact specifically for the latter day church and latter day saints. The warnings were not included in the record to point out to us faithful latter day saints how all of our neighbors that don't have the true gospel are failing. That premise is born of vanity and pride.
Mormon set out the purpose for including the failure of the Nephites in his abridged record. He said, (Mormon 5)
8 And now behold, I, Mormon, do not desire to harrow up the souls of men in casting before them such an awful scene of blood and carnage as was laid before mine eyes; but I, knowing that these things must surely be made known, and that all things which are hid must be revealed upon the house-tops—
9 And also that a knowledge of these things must come unto the remnant of these people, and also unto the Gentiles...
10 And now behold, this I speak unto their seed, and also to the Gentiles who have care for the house of Israel, that realize and know from whence their blessings come.
22 And then, O ye Gentiles, how can ye stand before the power of God, except ye shall repent and turn from your evil ways?
23 Know ye not that ye are in the hands of God? Know ye not that he hath all power, and at his great command the earth shall be rolled together as a scroll?
24 Therefore, repent ye, and humble yourselves before him, lest he shall come out in justice against you—lest a remnant of the seed of Jacob shall go forth among you as a lion, and tear you in pieces, and there is none to deliver.
It seems to have been abundantly clear to Mormon, even if it fails to take hold in the hearts of the latter day gentile church, that what is needed for the latter day gentile church is exactly what might have saved the Nephite nation. "Repent ye, and humble yourselves before him, lest he shall come out in justice against you—lest a remnant of the seed of Jacob shall go forth among you as a lion, and tear you in pieces, and there is none to deliver."