I disagree. buttons on screen or off as soft / hard keys means you still have the same amount of space for the screen, but if the keys are part of the screen they can disappear if an App wants the whole screen. OK, in reality most apps appear to leave the buttons there, mostly reducing them to dots but either way you haven't lost screen space, you got a bit extra screen for the buttons. And if you want to hold your phone upside down...
You can also rotate the icons if in landscape and it allows for a reconfiguration of the keys in the future.
I also think the design looks cleaner without a physical button on the front, or a menu and back icon permanently on display.