Why did Paul say prophecy was the most important thing the local church could have?
At that time the New Testament was not yet written; the Gospels, Paul's letters, and the written Word were unavailable to the early churches. They depended on gifted men to proclaim God's truth orally, so prophecy—speaking forth the Word—was the greatest gift for the local church.
Les explains that when Paul wrote to the Corinthians, there was still no New Testament. The Gospels had not been written, and Paul's own letters had not yet gone out as the Word of God. These early, primitive, apostolic churches were growing in the pagan world without the benefit of the written Word. Therefore they had to depend on gifted men who could teach the Pauline doctrines, not what was still coming out of the Old Testament, or there would have been pandemonium. Paul, by inspiration, recognized that the most important thing the local church could have was men who could proclaim the truth of God's Word without having it in print. So he told them to desire the gift to proclaim the Word of God. Les also notes that chapter 13 said the time would come when that gift would fade away; it is no longer valid because we now have the printed page, and we do not need men gifted to proclaim the Word without anything to go on strictly by God's power.
Follow after charity, and desire spiritual gifts, but rather that ye may prophesy.
More Questions
What does it mean to prophesy in 1 Corinthians 14?
The word prophesy in this context does not mean telling the future like Daniel or Isaiah, but simply speaking the Word of God and sharing it.