Years ago, I was at a grand opening for a leashless park. One of the top trainers in the area was there with his dog. As we stood outside the enclosed area watching people with their dogs unleashed in the parking lot, the main park area, and the leashless area, Kevin said "this is the stupidest thing I've ever seen. No matter how well trained my dog is, I would never let him offleash like this." His point was that no matter how well trained HIS dog is, he cannot guarantee that of every dog they meet. It doesn't matter that your dog will stay at your side or stay on the mat or stay at attention. That other dog could approach your dog and a fight ensues. And without a leash on your dog, you will have to stick your hand into the fight to pull your dog out. Shortly afterwards, we saw a woman walking through the dogs with a cat on her shoulder and he said "no that's the stupidest thing I've ever seen".
When I had Dog, we were on our front lawn practicing stays. She was in a stand stay. A dog that was walking down the street with its owner (unleashed) ran up onto the lawn and literally bulldozed Dog over. I had Dog on a leash and was able to lead her away as this other dog kept trying to knock her down. The owner was chasing along behind trying to grab its collar. Another time at a leashless park, I had just walked inside with Dog when a large chocolate lab walked up and peed on her. The owner stood there laughing. Dog was still on her leash - we turned around and walked out. Dog was an extremely well trained and well behaved dog, but I can't control the behaviors of other dogs or people so I make sure I have a leash on my dogs regardless of their obedience level.