/rest/pipelines API was added to get pipelines across organization. This API is not needed because of these reasons:

• It can only handle GET on collection, GET, PUT, DELETE on /rest/pipelines/:id is not possible as pipeline 'id' is not unique across organization.
• There is already search API that gives collection of pipelines across organization /rest/search/?q=type:pipeline. Adding new API just to get collection of pipeline across organization is overkill and adds to confusion as the API can't handle other HTTP functionality on that resource.