I think, just my opinion, that they are really quite unsure and need validation. If they convert (however unlikely) they have found some validation of their beliefs. Note, there are lots of people (I'm also thinking of you here, Melissa) who do not seek validation for your beliefs. You're confident about them, happy with the skin you're in. Also, leads to better relationships with athesists etc.
It's a topic nearly everyone has an opinion about--whether they believe one way or another--people are passionate about beliefs and non-beliefs! I don't believe people should prod and force their views on others, yet on the other hand there are quite a few people out there who are bringing up the topics via way of Hubs, forums, and questions to which responses are therefore given (some responses more respectful than others--for both the "believers" and the "non-believers"). It is a hot topic though--no getting away from that. I just wish it could be a healthy exchange of ideas rather than people attacking and criticizing one another. Sometimes this happens--many times it does not. One of my favorite hubbers to read has opposite beliefs than my own but I learn from him and enjoy the discussion.