When I used to just ask for a cover letter and resume, 99% of the people who interviewed with me were just awful. Now I tend to get good dev interns who want to be here and have similar drive and ambitions.
I'm not saying that a person who doesn't have any OSS experience, a blog, or a Twitter account wouldn't be a good developer, but then if they're not interested in OSS, teaching, doing things on their own (outside of work/schoolwork) or sharing, then they may not be a good cultural fit.