Drum brakes have a feature that makes them beneficial over disks (besides cost). Drum brakes are often designed to be somewhat self exciting. When the cylinder or magnetic servo engages the shoes and they grab the drum, the assembly rotates and tends to grab more aggressively. Think of it like a door stop. Push harder on the door and it grabs harder. In this way the servo doesn't need to be as powerful as one on the more linear geometry of disk brakes.
Drums also have better static holding power which is why they are often used in for parking brakes too. Many vehicles actually have small drums hidden within the rear disk units because a suitable disk caliper with parking brake couldn't be designed. Those cars with rear calipers equipped with parking brakes often use a pedal for the ebrake instead of a handle due to the force required to activate them.
The draw back, as noted, is the poor heat dissipation on drums that causes outgasing of the brake lining and hence fade.
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