If you read books like Influence: The Psychology of Persuasion, you learn that people are influenced by having to say something out loud. Commenting in the congregation forces people to publicly commit to their faith. This reinforces the indoctrination in a big way. It's one thing for someone else to tell you from a stage, it's something altogether different to hear it coming out of your own mouth.
Field service has a strong reinforcing effect as well. I believe this is the main reason for field service. The recruiting aspect is secondary.