Tinstaafl
"Not having a/c on a common QNH is worse. How can you have a meaningful comparison of altitude eg climbing/descending/crossing a/c, if your reference points are arbitrarily different?"
My point here is that in reality a large percentage of the traffic in any given area is using an aerodrome QNH or QFE i.e. it is departing or arriving an aerodrome in that area. So what you've got is a mish-mash of traffic on all different settings regardless of whether they are using regional QNH. So even if you can compare altitude of traffic using the regional QNH, how do you account for the other traffic in the area which is using aerodrome QNH/QFE???