I have to disagree here. I almost never use the “flat list” option because 95% of the time I’m not interested in just seeing things, or even modifying them in place; I want the ability to move things around. I also want a representation of my Binder hierarchy that’s different from the Binder/Outliner linear indented list. I’d much rather
- use option-click on the disclosure triangle of the root container to expose the entire hierarchy.
- select a container that’s “as far down as I want to go,” then use View->Outline->Collapse all to current level to focus on the levels I want on my corkboard
- Select all the containers from the root right on down to the level I selected in step 2 to get a multi-level stacked corkboard.
Not only does this give me the stacked corkboards I want, with the ability to move any item among them, I can use the “Navigate through groups” icon/ menu (You already know, 
but others might not). It’s a little menu icon up in the corkboard header that appears when corkboards are stacked, and clicking on it lets you navigate to any container’s corkboard.
Much more my style than the undifferentiated mass of cards I get from the “open as flat list” option.