Originally Posted by
AirNoServicesAustralia
I think you will find that in reality most pilots change to QNH when cleared to an altitude due to the workload issues that quite often occur during the phase of the flight that corresponds with passing through the TL. As controllers we obviously would prefer that they stay on 1013 until passing through the TL but from what I have been told and what I have seen this isn't usually the case.
Only becomes a really big problem when you subsequently have to reclear them at a FL, and they forget to switch back to 1013, which has happened to me before.
Not everyhere is like Australia. Speaking as a simple controller, I can understand why crews want the QNH set as they descend towards a variable TL with a TA that is often little more than 1000 or 2000 ft above nearby terrain (which is the situation over large parts of Europe).