Too often, web tiers are full of boilerplate that does nothing except convert a result set into JSON. A middle tier could be as simple as a function call that returns JSON. All we need is an easy way to convert result sets into JSON in the database.
PostgreSQL has built-in JSON generators that can be used to create structured JSON output right in the database, upping performance and radically simplifying web tiers. Fortunately, PostgreSQL has such functions, that run right next to the data, for better performance and lower bandwidth usage.
I certainly wouldn’t advise this in many (most?) scenarios, but I can see a time and a place where “cutting out the middle man” would be quite advantageous, indeed. Keep it simple. Keep it lean.