And if you're thinking "My dog is friendly, he wouldn't hurt anyone one on a flexi lead" - think about this: what if that dog who approached us was a friendly dog and we had an UNFRIENDLY one with us? That dog startled us and would definitely startle a dog who is easily spooked or reactive. Anyone who has struggled to train a leash reactive dog probably has stories about frustrating encounters with dogs on flexi leads or off-leash dogs that are poorly trained. You have to consider how your dog may make other dog's react, and do your best to keep your friendly dog safe!
Flexi leads DO have a time and a place. They can be useful for recall training or used as a good "free-time" leash at the end or in the middle of a walk if your dog is not yet off-leash dependable. But as a standard leash, they generally only get you tangled up and put your dog in danger (stepping into the street at the wrong time) or other dogs in danger.
Everyone needs to practice responsible dog ownership, regardless of your dog's temperament, and ensure that you understand the anatomy of a good dog walk!