The essential unity of belief is stressed. While the congregations were separate, they were all teaching the same religion. This was assured by Alma's position as leader for all of the churches.
Doctrinal: This verse gives a formulaic definition of what was taught in these churches. Rather than say that the gospel was taught, or that commandments of God were taught, or that the law of Moses was taught, we have "there was nothing preached in all the churches except it were repentance and faith in God."
Certainly the churches taught much about living the will of God, and could not have been limited to sermons about repentance and faith any more than the modern church could long hold our interest is repentance and faith were the only topics. The important keys here are the introduction of the phrase with the negative (there was nothing preached…except…) and the formulaic use of "repentance and faith."
Repentance and faith are keywords that form the basic understanding of the Atoning Messiah. First is the individualized rather than communal nature of the religion of the Atoning Messiah, and secondly is the emphasis on the faith/repentance/atonement/forgiveness that forms the basis of Christian worship. This contrasts with the communal atonement of the law of Moses. While it may be argued that faith is always a principle of obedience to the divine, the elevation of the necessity of faith is a Christian principle rather than an Old Testament principle. The exhortations to faith in the New Testament find no firm counterpart in the Old Testament. Part of this is the shift in the emphasis from communal salvation to individual salvation. The second is the decrease in the emphasis of the efficacy of performance (law of Moses) as opposed to individualized righteousness (Christ's transformation of the law, as in the Sermon on the Mount).
All of this suggests that what is actually being said here, is that these churches preached the Atoning Messiah. This is consistent with what Alma learned from Abinadi and taught to his first congregation. Thus the statement deals with the type of gospel being taught, which is that expounded by Abinadi and other Nephite prophets concerning the Atoning Messiah.
Translation: This verse inserts the denominational meaning of "church" into the description of Alma's church, precisely the definition I have argued against. What is it doing here? This is Mormon's abridgement, and Mormon is from a much later period when the divisions in religions have played our very differently from what Alma envisioned when he established the churches/congregations. I see this as Mormon's conclusion based upon his later understanding of the conception, rather than a textually accurate depiction of Alma's churches.