The Drone makes up the economy of the Zerg, being the gatherer of resources and builder of structures. The Drone is consumed in the process of building, or rather morphing into a building, a unique feature of the Zerg.
As with all other workers, the Drone, though limited, also has the ability to fight. So repelling enemy attackers with just drones is feasible. Combine that with the natural hit-point regeneration on Creep, and you have a small standing force ready to fight back in small skirmishes. Compared to Starcraft, no additional changes to the base unit have been made.
