An example of when both docking windows would be helpful is dealing with
heading levels. For example, from a new document, if I press Shift+Alt+Right,
the style Heading 2 is applied. I can tell what the style is from either the
Quick Styles control or the Styles window. Now, if I press Shift+Alt+Right
again, it becomes difficult to tell that I'm on Heading 3. I'd have to
remember what depth I am at or tell from the font, both of which are
unreliable when working in a complicated document. Otherwise, I have to
manually customize the list of visible styles in the Quick Styles or Styles
list, but this takes effort and wastes screen real estate on styles that are
easily keyboard selectable.
The Apply Styles (or Style Inspector) window is convenient because it always
shows the current style. Adding the Style command to the Quick Access Toolbar
helps sometimes, but it doesn't appear to be resizable. This makes it work
for headings, but for styles with longer names, it would still be helpful to
have a way to always be able to see the current style and be able to select a
new one, but without giving up too much horizontal space.