When working on a large document like a book (particularly if you are designing the book while you are producing it), you are constantly revising parts of the page, then going back to refine it as the design evolves. It would be great if you could mark up the Pages Panel with notes (like PDF markups) in the pasteboard like space around the spread icons. This would allow you to leave yourself or others on a team critical information about what is a placeholder and what should be reconsidered.