In his most recent post Brandon Savage looks at a practice that's becoming more and more common - using a REST API as a data source for a web-based application.
We were faced with an increasing number of data sources, including the coming addition of Elastic Search to the data storage system, and maintenance was becoming a problem. Thus, the decision was made to move our data layer to our REST API exclusively, removing all direct access to data storage from the web interface. This is the second such project I've been on where an external API has been used for the retrieval of all data in an application. It's a novel concept, but one that takes some getting used to to be sure.
He talks about some of the advantages of this approach including the ability to be more flexible with the actual technology used to create the API, the ability to write other applications on top of it and a glimpse into some of the process that's been going on as a part of the project.