It's been stated before, but I'll say it again here.
The Watchtower teaches:
1) Jehovah can read hearts.
2) Not all the inhabited earth will be reached with the "Kingdom Message" before Armageddon.
What's the point of the preaching work, if Jehovah can read hearts and doesn't need JW's to go out and expose people's heart condition?
The point is to keep the JW "publishers" on a treadmill, so that they can be judged by how fast they run on the treadmill, and to keep them from having the time to have a real life.