The REST API for GeoServer is popular, but not well maintained, collecting a large number of outstanding bugs. Many of these complain about functionality and lack of documentation. The large number of bugs (API called correctly but produced an error) may be producing more requests for documentation (developer assumed they called it incorrectly, and asks for improved documentation with an example that works).
Internally the REST API is written using an early java library called "restlet" (http://restlet.com/). There is a desire to migrate to spring rest api which is annotation driven and better supported. The risk in performing a migration to Spring MVC is introducing more bugs than are fixed. This is somewhat offset by having a larger pool of developers familiar with the codebase and the technologies used.
For planning coordination and details see: