I don't think someone would want to always carry a screwdriver or keep one nearby everytime he uses his calc then have to unscrew the battery case every day to remove the battery.
That said I have absolutely no clue why those batteries are drained completely when the calc is not used and becomes in Standby mode, when my Nintendo 3DS energy remains intact even after 6 months of non-use... It's almost as bad as the iPod Touch.