I agree with the previous posters. The d2d work is ineffective. It is a control mechanism.
Another point, imho, that shows this is the pioneer school and textbook. If your main concern is reaching as many people as possible with the most effective message why would you only let the elite go to the special training school and receive the special training manual? How does that help the worldly people?
Also, District Convention releases. The releases at the DCs are touted as the new great thing to bring people into the truth. No one is to know what's being released before their Convention and the Conventions around the world go one for months and months. If the whole point was to reach as many people as possible with the newest and best information the WTS would send the publications out with some information as soon as they were ready.
The above instances aren't designed for the field - they are designed to motivate and emotionally control the members.