I used one when stock controlling pup through Sheep, and it was fine, its just being responsible, have walked the dog up a cycle path pup was fine, when seeing an approaching dog, just bring it in, and grab the lead.
but i do know what you mean, as one day i was walking the dog on a short lead, on the other side of the road came a Woman with the extended lead short, but obviously not locked, her Poodle ran across the road to get to my dog, she was lucky, as its a busy road, the poodle even managed to pull her off her feet, and she fell over.
Its easy to click it to stop it running, but half