Most dog trainers will tell you that in order to truly handle your dog, you must have a 4-6ft sturdy lead. This lead should allow you to keep your dog under your control at all times because we all know, the further away you are from your dog, the least amount of control you have over your dog. With a retractable lead, your dog can be extended 10 to 30 feet in front of you. If they were to suddenly bolt across a street, you have given them that much space to move and put themselves into traffic. They see and interact with other dogs, people and small animals and are far enough away from you to get into trouble. There are reports of dogs getting hit by cars and kids on bikes because they are not street smart enough to know to watch for these dangers.
The lack of control of the dog can lead to the cord burns to folks as their dogs get tangled. I have been burned as someones dog walked behind me and the owner tried pulling in the lead. As the cord pulled across my lower leg, I was burned through my denim jeans leaving a red welt on my skin. The owner had already started the damage and I was on my own to get out of the mess before additional harm was caused. To be honest, there was nothing she could have done other than use a better lead to control her dog. Her tiny Rat Terrier was excited to greet me and my own pup. I had the better (and traditional lead) and was able to untangle our pets while all she would do was stand their while I did my best to save my skin.
If you feel its convenient to allow your dog to wander further from you and do not feel the risk of injury to you, your dog or the people you interact with while walking then do a Google search. Search retractable lead dangers and read all the stories from pet owners who have severed fingers or scared their legs and arms of themselves and their children. You can even ask my friend Berhan about the 9 inch scar on her leg from her Chihuahua walking around her while waiting to cross the street.