The purpose of the curse was to separate the two peoples. This was done so that the righteous would not intermarry with the wicked and thereby adopt their wicked ways. In order to understand the purposes of the Lord in this regard, it is useful to examine a similar situation as found in the Old Testament. The Lord commanded the children of Israel not to intermarry with the non-Israelites in Canaan:
if ye do in any wise go back, and cleave unto the remnant of these nations, even these that remain among you, and shall make marriages with them, and go in unto them, and they to you:
Know for a certainty that the LORD your God will no more drive out any of these nations from before you; but they shall be snares and traps unto you, and scourges in your sides, and thorns in your eyes, until ye perish from off this good land which the LORD your God hath given you. (Josh 23:12-3)
Just as the Lamanites were to become a scourge to the Nephites (v. 25), the Canaanites were to become a scourge to the Israelites. The peoples which inhabited the land of Canaan while the Israelites were wandering in the wilderness were very wicked. They practiced homosexuality and beastiality:
Thou shalt not lie with mankind, as with womankind: it is abomination.
Neither shalt thou lie with any beast to defile thyself therewith: neither shall any woman stand before a beast to lie down thereto: it is confusion.
Defile not ye yourselves in any of these things: for in all these the nations are defiled which I cast out before you
And the land is defiled: therefore I do visit the iniquity thereof upon it, and the land itself vomiteth out her inhabitants.
Ye shall therefore keep my statutes…
That the land spue not you out also (Lev 18:22-28).
The people were so wicked that the Lord brought judgment upon them via the armies of the children of Israel. The children of Israel were instructed to kill everyone, even women and children. This was the only way to cleanse the land of iniquity and keep the Israelites from learning of their wickedness:
But of the cities of these people, which the Lord thy God doth give thee for an inheritance, thou shalt save alive nothing that breatheth:
But thou shalt utterly destroy them; namely, the Hittites, and the Amorites, the Canaanites, and the Perizzites, the Hivites, and the Jebusites; as the Lord thy God hath commanded thee:
That they teach you not to do after all their abominations (Deut 20:16-8).
The Lord understands the great influence a wicked and idolatrous spouse can have on a righteous individual. With all the wisdom Solomon had, he was not wise enough to stay faithful to the Lord when his many idolatrous wives began to influence him, when Solomon was old…his wives turned away his heart after other gods: and his heart was not perfect with the Lord his God (1 Kings 11:4).
The other reason to separate the Lamanites and Nephites was that the Lord had promised that all those who were living on the land of promise that would not keep the commandments would be cut off from his presence. This is what happened when the prophets of Nephi, Jacob, and Joseph left with their families to start a new life. The Lamanites then had no one to teach them the things of the Lord. See verse 20.