From the January 1941 edition of Popular Mechanix:
Dog Leash on Spring Reel Plays Fido Like a Fish
A leash attached to a spring-operated reel is a new wrinkle for dog owners. Encased in a light but sturdy aluminum housing, the reel carries twelve feet of waterproofed leash strong enough to hold a great Dane yet light enough for use with a Pekinese. A hook forming part of the housing allows the owner to hold the device securely, while the dog is free to run for a distance of twelve feet. As he returns nearer to his owner, the spring reel winds up the leash cord to prevent it from becoming tangled. A button-con-trolled lock makes it easy to stop the dog at any time as he is running out the leash, and to keep him in hand.
As a dog walking tool, I consider the retractable leash a nightmare in the hands of most pet owners, but as a dog training tool, the retractable leash is a very good long line, as it's easy to get it out of the way as you teach recall and work with distractors.
Jay Jack has a $10, 15 page e-booklet on how to use a retractable leash that covers how to hold it for control, how to make sure the reel never leaves you even if the dog jerks very hard, why a web retractable is better than a string retractable, and how to use rewards and a retractable leash to teach heel and a few basic commands, etc.
Bottom line: Retractable leashes have a place in dog training.