I tried this once: we had a page with info on what would become a project but wasn't at the time. Then I created the project and figured I could just "move" that wiki page to the location for the project wiki page. It was allowed and worked but things were not working perfectly fine afterwards i.e. I think that wiki page was not appearing under "Projects" in the hierarchy. Moving it back did not fix the issue. I had to go in the database to fix things and manually delete some records.
That's an edge case granted, but if this is not robust, is there any reason to allow moving docs right under "Projects" in the first place?