I can't begin to express how useful the ability to assign a category to a Page would be. My site is being used for content management as well as blogging. A lot of the content is static and so it's in Pages. If I could assign a category, I could group and arrange the Pages in cool and useful ways. Since I can't have categories, I rely on some very old and not too stable plugins to help me manage my content.
Parent Pages aren't the same. The page parent becomes part of the permalink structure, and that's not what I want. If Parent Pages had been available when I started, I might have gone that way. But they weren't. If I tried to change the hierarchical page structure now, my permalinks would be impacted. Since the static content drives a significant percentage of my traffic, I don't want to mess with the link structure.
I just want to be able to group and arrange without messing up my permalinks.