Dog rescue shelters are full of staffies and staffie crosses. I wouldn't chose a staffie as a pet, but have known child minders and foster families whose staffies were bomb proof around children. Most of the "staffies" i came across in the last 10 years as a sw, were not full staffies, but crossed with some type of bull breed, so prospective adopters have no real idea what they're taking on. I grew up with a bull mastiff, a gentle giant of a dog. As others have said, generally one look at the owner will give an indication of the way the dog has been (or often not been) trained.
I sympathise with those who would like all dogs on lead in public spaces. It's already the case that dogs are to be on lead near children's play areas. Dogs need off lead exercise and if they're properly socialised and trained, should cause no problems to other dogs, or people. I believe everyone who buys/rescues a dog should have to attend at least basic obedience and responsible dog ownership training. The kennel club good citizenship courses are excellent. If owners can't afford the average of £6 a lesson for the 6 week initial course, they can't afford a dog. Dogs should all be chipped, and some form of registration brought in.
I am pleased to say that a local "professional dog walker" is no longer bringing his unruly packs of 5 assorted dogs to our local park. The community policeman knows all the regular dog walkers, and we all complained to him and the dog warden about the fact this chap had no control over the dogs. He didn't know them, they didn't know each other, so he'd pitch 3 or 4 times a day, with different groups of between 3 - 5 dogs, open his van doors, and let them run. He never picked up after them. Well done that community bobby and dog warden for doing whatever they did.