it doesnt really matter if these little front engine cars have disc at the rear or drums, their front brakes have ample stopping power.
these cars require lesser breaking force at rear end, and drums do the job fine, y increase the price by putting in a disk and then you have to distribute the break force even further.
even the 7th gen city has rear brake drums for non hybrid.
edit: however in the case a car is full with passengers at the rear(3 or 4 adults), the disks would make a difference,