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.
I prototyped something exactly that way, including using Couch. It was really great, and quick as you describe, but ultimately still required a proper middle tier since the gap between a prototype and something deployable is often a lot bigger than one realized, and I ended up having to do a lot of work to get one situated in the middle. Though that ended up being an enterprise app, which adds complexity.
Yep, enterprise apps definitely fall under the "serious" umbrella :).
If a middle tier is a foreseeable requirement then this architecture does lose a lot of its charm.
I do think there has to be a way to easily strangle out the prototype but I haven't done much thinking about that yet. Maybe start reading from a pouchdb instance and proxy writes through a middle tier?
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.