For really small projects, a proper backend is not always necessary and talking directly to the db can speed up development. I wouldn't dare do that for anything serious though.

Couchdb is also a really nice way to do this, since it provides you with an extensive (an extensible) rest api out of the box, serves your site and even gets you a nice heroku like deployment workflow with the Couchapp project (https://github.com/couchapp/couchapp). Sticking it behind varnish is really easy too.