You can do a lot of damage to a leg with a flexi. A dog runs the wrong way, it gets wrapped around a leg and pulled... ouch. It's even worse if the person is wearing shorts.
I watch people walk massive dogs on the tiny string, it's an accident waiting to happen.
Then there are the people who have problems controlling their big dog so they wrap the leash around their wrist and/or hand. When that goes bad it can break bones!
Long lines deserve our respect as well. I have a video of a friend of mine being pulled off her feet and dragged on the ground by her ankle while handling Bullet's line during bitework, I'm still looking for it. Thankfully she wasn't hurt, and Bullet didn't even notice!
Yesterday I was running Lexus with another dog and I had her dragging a Gripper long line. A Gripper line is great for when you are holding back a large dog in training. It has long lines of elastic through the webbing to provide grip. I love mine! That is, until puppy runs around my leg while dragging it. I was wearing a skirt and sandals. Seeing what was coming I jumped out of the way but the end still managed to catch my ankle and flick around, taking a few layers of skin with it. Nasty.
I don't recommend it!