The exchange of letters requires that there be some at the higher levels of both armies who could read and write. It also preserves the ability of the leaders to exchange information without fearing treachery upon their persons. Since the messengers were not politically important, there was less reason to break the truce required to exchange such messages.
Mormon begins by noting that Jehovah is on his side, and that Jehovah will see them destroyed unless they leave. They would not only be temporally destroyed, but eternally destroyed and consigned to hell.
In spite of that opening statement, Mormon moves to the more likely scenario, noting that “as ye have once rejected these things, and have fought against the people of the Lord, even so I may expect you will do it again.” Mormon remembers that Ammoron is an apostate Nephite who had joined with the Lamanites.