There are many different situations when as an Owner/Contributor, I would like to collaborate on notes or a whiteboard with my colleagues who only have Reviewer/Viewer access to my workspace.
For example, there may be an upcoming brainstorming meeting, or PI planning session where many different roles come together. It would be helpful to allow those users to prepare their input in the shared document.
It would be useful to allow collaboration on specific documents, as opposed to any document in the workspace.