The ones that I think should be banned are the ones with the cord as the leash rather then the ribbon type. It's the cord that mainly cause the problem with slicing people or giving them rope burn, the ribbon ones are much safer.
As to the rest I think they have their uses and used to use them extensively in the city with my two former dogs. My two were not allowed to run up to anyone on the Flexi, and they never ran into the street on it. I knew how to use the break and the break never failed even after years of daily walks, I think it's because I had real Flexi lead and not cheap, you get what you pay for, knock off brands. They allowed my dogs a bit of perceived freedom and I used the leads responsibly.
My main problem is that people more often then not do not use the lead responsibly, they let their dog run everywhere, run up to people and dogs, wrap the lead around people, and don't seem to know how to control the dog using the lead. I've also witnessed a person I know grab the cord to check the dog, I told her that was a bad idea and that if the dog ran she'd have a rope burn or worse, I then seen her do the same then the next day. Thankfully she was dog sitting and the people she dog sat for have moved so she's no longer endangering herself.
The cheaply made retractable leads are also a pet peeve of mine, the breaks have a tendency to fail, and they can also decide to stop retracting, both endanger the dog. If a person must use one I really wish they'd opt for a good brand.