I think a lot of people use retractable leads because they think that they will tire their dog out by letting the dog run circles around them, instead of just taking a longer walk and teaching the dog how to walk properly without pulling. I was absolutely guilty of this when I was a new dog owner. I have two instances of near-injury with these things:

1. When I was learning how to walk my dog, maybe that first couple of weeks, I tried this because I wanted to walk the dog less (he's really my husband's dog) and have more time for cycling. There was a moment when we were in a park, dog saw a squirrel and took off at breakneck speed, all the while the leash is letting out more line. This is a 50-lb dog running at top speed. Me, 110-lbs and standing still. I had to make a quick decision to either hold onto the lead cartridge or let it go -- because either the dog was going to pull my arm out, or if I let go the cartridge would get caught on something and the dog would get hurt at the neck, or the dog would stop running and the cartridge would retract and hit him in the head. I quickly fixed the lead length as best I could and let go. Thankfully neither of us got hurt.

2. About 6 months later my dog was impeccable on a 6' leash. While out on a walk, we slowed to exchange greetings with a random other dog walker walking two small hyper dogs on retracto-leashes coming the other way. One of the little dogs proceeded to run circles around my dog, resulting in two wraps of very thin line around my dog's neck. Thankfully I was able to untangle him before the line got tightened around his neck. Henceforth, when I see someone with this type of lead, I will go out of my way to stay out of their range. My dog already had enough trauma before we adopted him, he doesn't need any more from people who don't know how to handle their dogs. This is why I hate retractable leashes.