I think Rebel8 overstates the issue of door knocking's effectiveness. Goodness knows there have been enough threads on this forum demonstrating that the success rate of the JWs door-knocking is astonishingly low.
It depends on what you take to be the objective of the door to door preaching. In terms of making converts then it is very ineffective. But within the Witness community, in terms of building solidarity, a sense of common pupose, and a continual affirmation of each other's faith in the organization and its enterprise, it is an extremely effective tool. What could bind people together better than coming together to present an unpopular message to an outside world that meets with overwhelming rejection?
When I stopped going out in the ministry it was less about the remote possibility that something I said might affect a householder. It was more to do with feeling guilty about giving false encouragement to others in the congregation who will inevitably view the attendance and participation of others in the ministry as a sort of social proof that it's the right thing to be doing.