I don't like the logic of 'When will you ever go through a thunderstorm at max gross weight?'
These aircraft will fly for decades and will age through the process of thousands of pressurization cycles, firm landings, and wing flexes. Their initial structural strength will deteriorate over time. Initial certification testing's purpose is to assure structural integrity over the design life of the aircraft.
Design tests are mandated to prove the design concept under actual physical stresses. This isn't a Boeing vs. Airbus issue, but one of aviation safety.