I believe people who come into the JW's as adults are looking for an answer to their problems - and I would bet that most of them have little to no knowledge of the bible. This is why they are susceptible to the twisted doctrines and teachings of the JW's. They see people who seem to know their bible, have an answer for everything, and are very HAPPY - so they decide this must be the way.
I also believe that anyone who has really studied their bible and has a basic understanding of it's message would NOT be drawn into the JW organization as an adult.
And JW's assume that most people don't know their bible - which is why they like to FLAUNT their knowledge about it at the door. But when they run into someone who really DOES know their bible and is ready to defend their beliefs with scriptures - well, the JW tucks tail and runs. So much for the "ready to make a defense" scripture hu?