In general, I agree with you, but there is a sense in your comment that if people are well trained, experienced, and pay attention, mistakes won't happen. This is not actually the case, even well trained, thought-full, experienced, and focused climbers make mistakes. Equipment such as auto-locking belay devices can definitely help reduce the consequences of those momentary lapses. For instance if a belayer suffers rock-fall, bee-stings, heart attacks, dog-bites, a strange twist in the rope, etc an auto-locking belay device would mitigate potentially fatal falls.