Currently RESTBase exposes endpoints get the sections and save the edited sections of the article. This API was built for VE section edits to save bandwidth by not transmitting the whole article to the client. However, the API is currently not used and it's not clear whether VisualEditor team has plans to use it in the future. If not, we should deprecate and remove the API which will also help with Parsoid PHP porting project since section offsets calculated in Parsoid will be incompatible between PHP and JS.
Question for the VisualEditor team - are you going to use the section editing API?