Several decades ago a researcher named Solomon Asch conducted an experiment on conformity to determine the conditions under which individuals will deny their own senses in order to conform with a group.
If you view the video clip, you will notice in the first two scenarios presented that those who conform tend to do so for one of two reasons: one, they doubt their own judgement in the face of unanimous opposition or two, they still believe they are right but conform in order to avoid conflict within the group.
The key to total shunning of DFd and DAd individuals, as well as constant pressure to avoid "bad associations" is based not only in the conformity behaviors mentioned above but due to what Asch discovered in the third scenario presented in the video, when the test subject has a single confederate who agrees with him. The key to conformity within a group is unanimity. Just one dissenter can release a thinking person from the mental bonds of group conformity, giving him free license to think for himself. This is incredibly dangerous to a high-control group such as JWs, who pride themselves on the unity of their worldwide brotherhood.
If they allowed free association with those who do not conform to the group, the majority of even the staunchest conformists would feel free from the obligation to conform. There would be A LOT more of us and a lot fewer of them. (As a side note, this is one of the values of being a fader who still has some association with members - we can plant seeds if we're careful; a DFd or DAd person has no contact.)
Here's the video:
http://www.youtube.com/watch?v=TYIh4MkcfJA
Please view the clip and comment on my comments, and share the Asch experiment with your friends. (you might want to look up Stanley Milgram and Philip Zimbardo while you're at it...oh, and Lifton's thought reform research...etc.)