Aerodrome QNH last night: 1018
Regional QNH last night: 1005.
That made RQNH about as useful as tits on a bull to me on an IFR flight at night into an aerodrome in a valley without an instrument approach. I want to know my actual terrain separation thanks. Nearly as bad is having to use a plethora of aerodrome QNHs while travelling from A --> B. I'd rather see an area QNH that isn't the lowest but is within (say) 3mb of the area's aerodrome QNHs ie ~100' of max. deviation.
Oz uses 5mb ie the area QNH has a max deviation of 5mb from the actual QNH anywhere within that area. That gives a 150' margin of error. Still acceptable I think. If there is a greater deviation then the area is subdivided as necessary. Works well and elimates the UK problem of the artificially low Regional QNH setting putting you inside a CTA step. All aircraft outside the circuit use the AQNH, circuit/approach traffic use the AD QNH.
This allows en-route traffic to utilise a common QNH for comparison & separation AND allows flight past airstrips/aerodromes/CTA without being too far different from local.
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?