I'm inclined to agree that airfield QNH is more useful than QFE. The point of the post was to question the wisdom of aircraft in a small area using different QNH settings.
Why would this be an issue? If the aircraft are talking to the same ATSU who is offering them either a control service or an advisory service for separation, they will use the same QNH as given by that ATSU. If they're not, what hazard arises from the use of different QNHs?