I just wrote recalc today as a demo so people can extrapolate what they might do with Hyperbole (calc was handy in my head since I wrote it for Ober, the Java version of the Oberon environment).
WIth buttons that restore the point to where it was when you clicked, you can make them operate on the current selection (it looks like Hyperbole might not save the region though so maybe more function advice is needed for that). This makes reusable "menu bars" possible:
<recalc> <cut> <copy> <paste>
(changing recalc to find the table at point instead of using the location of the button).
Highlight a region and shift-middle-click on cut/copy/paste to do that action on your selection.
This also lets you use button bars in other frames (acting even more like a real menu bar).
Buttons could also parse contextual information in the button buffer (and/or the active buffer), something like org-mode properties, that could influence how the buttons behave. Other buttons could edit this information to give you document-based behavior. For instance, a button-buffer (like the menubar) could have behavioral information like this:
<indent>: 4
<justify>: left
Clicking "left" could toggle it between the different types of justification (left, right, center, full) and clicking on <justify> could justify the current paragraph. Clicking <indent> could change the indentation of the current file (if it's a program, it could analyze for the current indentation and then change it to the desired indentation).
Just thinking out loud here -- this is the sort of thing Ober did and Hyperbole looks like a framework that makes it not too hard to do that for Emacs.