Envira Gallery's Adobe Lightroom Addon that we launched last week utilizes the REST API

http://enviragallery.com/introducing-adobe-lightroom-to-wordpress-with-envira-gallery/

Although it's a premium addon, the REST API was essential to allow users to create WordPress galleries straight from Adobe Lightroom and keep that in sync.

I believe the low number of plugins in the repo is because devs are eagerly waiting to see what happens before putting all their eggs in a project.

I honestly wish that the REST API was here even earlier so we didn't have to build a custom API for our OptinMonster SaaS which is powered by WordPress.