I note that in your description of atheists, "quiet and nonhostile" go hand in hand, as do "vocal and hostile".
Daniel Dennett is a very vocal atheist, yet I find it ridiculous to consider him a hostile person. Further, one might compare the hostility that atheists show theists to the hostility that theists show atheists for some needed perspective on "hostile atheists".
I was specifically thinking of the worst group of all, the atheists of r/atheism who are both very vocal and very hostile. For an issue like this, there's hostile people on either end of the spectrum and being vocal helps makes them more so. A quiet and hostile person isn't particularly threatening and neither is a vocal and nonhostile person. I was not trying to suggest that being vocal alone makes someone hostile.
http://blip.tv/tech-love-live/osb09-donnie-berkholz-assholes-are-killing-your-project-2464449
This is specifically about why it's important to get assholes out of open source projects, but it applies in general. It includes an analysis of the social cost of keeping people around who frequently make other people unhappy, and in particular a way to balance the social costs (distraction, people doing much less work or leaving, useful volunteers not joining, assholes recruiting other assholes, etc.) of assholes against the useful work some of them do.