QNH has far more importance for IFR aircraft into major ports for instrument approach reasons, en-route whether you are plus/minus 5 hpa is not going to have much affect on safety at all. At the bottom of a Cat 1 approach with other errors combined 5 hpa could mean reaching terra firma a lil prematurely. Add that an IFR aircraft may not fly level from transition altitude to touch down allowing any altimetry error catches by ATS. Easier just for ATS to pass and confirm QNH for all aircraft and not be selective I suppose is what they are aiming for.