How many of us (in the UK) fly around the visual cct with QNH set?
Similarly, how many of us transit underneath a TMA with the local airfield's QFE set?
How many of us have tried to set the Kabul (6870ft AMSL) QFE?
Why do we have a mixed transition altitude in the UK (mostly 3000ft but 6000ft under a number of TMAs)?
What does it matter as long as everyone knows what datum they are flying relative to and the controllers can do their maths when mixing 2 or more pressure settings?
Personally, I think the mixed use of pressure settings can be a recipe for disaster. However, surely we can just bite the bullet and get on with it.
PS. How many feet equate to one millibar at 35000ft when the temp is ISA -10deg Celcius. Anyone who uses the 30ft rule will be in for a shock.