Alma 3:7-8 expands upon 2 Nephi 5:21-22 which similarly indicates that the reason for the skin of blackness (the 2 Nephi 5 term) or the “dark” skin (the Alma 3:6 term) is to create a barrier between the Nephites and the Lamanites to prevent intermarriage.
Alma adds the information that the reason for the prevention of the intermarriage was to preserve the Nephites (v.8). This preservation comes by keeping the Nephites from believing in the “incorrect traditions which would prove their destruction.” Zeniff gives us the best picture of these “incorrect traditions” in Mosiah 10:12-18. In that recounting, Zeniff reiterates the struggle between the brothers, and couches the outcome in the perspective of Laman and Lemuel, who felt slighted at the loss of their rightful rule of the family.
In spite of this consistency in relating the natured of the incorrect traditions, how is that these stories would so influence the Nephites that they would cease to be Nephites. How would a Nephite renounce their very foundational myth for the foundational myth of their enemies (using “myth” anthropologically here in the sense of the story that gives meaning to the origin of the people)?
The real answer is that while everything might boil down to a simplistic story of differences between the original brothers, the reality of the five hundred plus years of their existence in the New World has moved them beyond a simple argument of right of rule. The danger that the Book of Mormon prophets preach against is not the problem of origins, but the attractiveness of culture. The adoption of what have become defined as Lamanite lifestyles would clearly lead to the destruction of Nephite ideals. Indeed, as has been discussed the complex involving kings, social stratification, and fine clothing is precisely the type of danger that could destroy the more egalitarian nature of Nephite culture.