Why do you have to wait until the FMGC entered TL to change? Change to QNH/STD when your SOP's dictate, not the aircraft.
Secondly, If you are bumbling along at FL55, you will get occasional notifications to set QNH (when it's not required) as the aircraft ends a few feet below FL55. After years of seeing this notification, people may ignore it when it's slightly more critical.