Volume:
You probably donīt want to have the electric load during takeoff run either. Just like with packless takeoffs on conventional aircraft.
I've gotta think about this in terms of "special case" high-altitude fields like Bogota, Quito, La Paz (4000 m altitude, 4000 m runway) and some Tibetan fields. While aircraft performance is enhanced by "packs off", I'd think some pressurization would be better for pax & crew alertness.