The argument that they give handlers control was obviously not the case here as the dog went missing trailing the lead. It got caught in hedgerow and it was only by hiring a company with a drone that the dog was found. He was 10kg lighter and had been eating twigs.
Dogs and people can be tanged easily and, when combined with owners not paying attention to their dogs can cause serious injuries.
Dogs should either be on a short lead, on a long line or off lead if safe and under control. Good lead training is essential for dogs and handlers and means that there will never be a need for an extendable lead.