U don't need a big bomb or a big truck. Heck, you don't even need to kill anyone. But if u can get an explosive, no matter how small, close enough to an important building or a person and detonate it, it would cause mass panic and fear.
You might think it's dramatic and over-reacting, but this is what terrorists thrives on, our belief that everything is fine and dandy.
If today a drone flew into a school and detonated a small explosive, even if no one is killed or nothing serious is damaged, would you open the school the next day and resume it as per normal?
Just imagine the fall out from one small event. Now imagine ten or 50 drones on auto pilot with small bombs all flying to the same important location.