The cage is just a pain to get around. If you get in an accident it's tougher for the people in the rear to exit. Also most rear bars aren't padded, so the risk of injury to your head goes up to rear seat occupants.
I wouldn't personally ever tote my family, friends around in a caged car except for one front seat passenger.
Low back seats w/o headrest, you need to incorporate a headrest into the cage itself. Drop it down off the main hoop.