I think you underestimate the value of a giant community interested in reading about stuff in general. They are the consumers of knowledge and content and are there to consume pictures, videos, AMAs, etc. While I don't always want to read AMAs, the most popular do make it through to my front page or I can see what's going on when I feel the urge, without ever changing sites. That value proposition alone is why I don't think I'd regularly use a dedicated AMA site.