That's not an argument against it. It doesn't matter if they suvive. It matters if they're discovered. If there are no rats/hummingbirds/whatever near your keep, when one shows up you pay attention.

For an example in our world, you don't need to be able to kill a spy drone to sound the alarm that a spy drone is flying around, because there normally aren't spy drone hanging about. If similar drones became incredibly common then you wouldn't know it's a spy drone or not. If you keep the numbers down, you know it's an issue when one is there.