In reply to Andrew.

Please see this post https://make.wordpress.org/core/2015/09/21/wp-rest-api-merge-proposal/ as it explains that with the Infrastructure part of the API added to WordPress, it enables developers to:

Merging the infrastructure now would allow third-party code to begin using the API to build upon it, including migrating from existing custom code. It would also help to increase developer confidence in the API (as it represents a commitment by the project towards a REST API).