Sure. My counter-argument is that for multi-tenant hosting like tumblr where millions of people keep their personal blogs, keeping posts in a database is common

Sure. My counter-argument is that for multi-tenant hosting like tumblr where millions of people keep their personal blogs, keeping posts in a database is common sense because they're a known quantity. Backup strategies, optimization techniques, operations procedures are all well known and battle tested. Also, creating content using a web-based editor that saves to a database is very easy for not-so-technical people to wrap their minds around. Static site generators? Not so much.