Why should believers leave just because not everyone here is also a believer? We talk about many subjects, most of them not related to belief in God. As ex JWs we all have a lot of things of mutual interest, so why wouldn't you just engage in those threads? Is everything an atheist says somehow tainted because we don't believe in God? Is your faith so weak that you will lose it if someone challenges your thinking?
As a believer you have many choices. You could simply avoid threads that discuss whether God exists, or you could read those threads with an open mind and not participate, or you can choose to debate your belief in God. It's entirely up to you. So why leave and miss out on the things that are offered here just because not everyone believe the same as you?
Personally I enjoy talking with everyone, believer or not. If I wanted to just be with people who all believe the same thing as I do I would have just stayed a JW.