Good point. All my dogs (5) have limited access to my back yard. They have their own area which I built for them myself. We also walk them so they get use to being around other dogs and people. Would I let 4 of the 5 off the leash, the answer is "hell no".
My PB mix stays on a leash when we are out also. He's such a sweetheart, but I don't believe I could predict his behavior in every possible scenario, so I do the responsible thing and keep him on a leash. He has a big dog yard at home, but he spends a good bit of time indoors.
I know he is protective of my MIL and myself and I do not always anticipate what he considers a threat - for instance, I'd have never guessed he'd feel the top of the artificial christmas tree was a threat :-) Anyway, since I know he can behave agressively if he feels he needs to protect us, it is my responsibility to manage him accordingly.
He doesn't have much tolerance for agression from other dogs, other than that he is the perfect pet. He understand MIL is not too steady on her feet and always clears out of her path. If she has him on the leash he walks with baby steps so as not to tug on her. He treats cats, babies, toddlers, rabbits and birds with tolerance and care. He is happy of heart and affectionate to all things he does not believe are about to hurt his pack. In other words, he's a better companion than many people I kno.