Drones are good as auxiliary weapons, rather than as the backbone of a fleet.
A bunch of drones can overwhelm PD and tip the scales of a major battle. Your enemy can't see how many drones you have with your fleet, and so may likely be surprised by them.
The zero maintenance is also in their favor when an empire is at peace and can't maintain more ships.
yeah, but only once since you can't recover them. I sure wish that spaceyard ships could build units (but nothing else) while moving. This would let you replenish your units while on the run.