Repair drones are for some specific builds which used Diamond Shell. Here's some of my builds using Diamond Shell:
Repair Drone's aren't really suitable for other torsos since they don't have much resistance especially for God Mode Torsos. Diamond shell + Energy/ Heat Shields with a decent resistance ( 20 to 30% resistance ) + Kind of build ( mostly heat or knockback mech like what is shown earlier)+ Repair drone ( Use the mark 2 if available and fuse it to 4) are mostly the builds I see. See,Diamond Shell may have low hp but with a resistance module + it's own resistances is really a good combo considering it lowers the damage of the enemy plus the amount of healing the repair drone gives. The only times these builds are in troublesome is when they are fighting against any Swarmers.