It's true! I remember even back when I use to go door knocking (about 10 years ago), the instructions we would get if someone wanted actually discuss doctrine was to say goodbye because they were most likely "apostates". Always thought that was weird, I know realize the truth is that if we were to discuss doctrine it would have been us seeing TATT and not the other person joining us.
If you think about it, what's the point of just reading the bible and studying it as a JW. You can't draw any other conclusion besides what the GB teach and think. You're better off just reading their literature and looking up the quoted scriptures. That's exactly what they want and exactly how it's set up.
As for defending their beliefs, I agree. They know they can't put a fight up and with so much on the internet that shuts down what they believe a JW will leave the door at a loss.
In the past, know one had access to the internet or their history. A JW would teach and argue what they were told to believe and the history they were told happened. The householder had no other information or nothing to present. That's all changed now. This is why they are now told just to leave because the JW will be leaving the door scratching their head thinking, "This wasn't how the Thursday night demo went?"
Even in public witnessing we are specifically told not to discuss doctrine issues. If someone comes up asking to direct them to the website.
It's sad but the preaching work is actually probably starting to make them lose members vs gain them.