@Cernel I often used the highlight all to help avoid forgetting units to move. I agree it's more useful when learning maps and eventually does not get used as much. That is in part why there are hotkeys. It's analogous to a keyboard, at first you are looking for a key and then pressing it. Eventually you learn where it is and you no longer look for it, you just press the right key without looking. Same thing for hotkeys and buttons. A new user will not know about the hotkey, they hover and learn that "Oh, 'F' will highlight all". They then stop using the button and instead use the hotkey. Flags having a hotkey is perhaps overkill, but play testing it, it does complement highlight-all.
@Cernel for a feature request, recommend waiting a few days for the new buttons to land and the updated flag behavior to be available in the prerelease. Please give it a try, and I'd be really happy to see any suggestions there.