Human aggression and animal aggression may be two different things, but when the animal in question has an owner who loves it, the situation is apt to become messy and lines get crossed.
Pit bulls were bred to fight dogs. Any aggression towards their handler was received with a bullet. They weren't bred to attack humans.
This is probably the second most popular argument in favor of pits, but I wonder how valid it is anymore. First, we're a long way from the old-time dog-fighters, who presumably had some standards of breeding and behavior, back before fighting dogs became a crime and a big business. Back then, I can imagine that the dogs were relatively well-selected for docility with humans, and that their dog-aggression was some sort of enhanced fight drive, in which highly confident dogs were encouraged to fight other, equally matched dogs. I doubt this is the case today. For the most part, I think, pit bulls and other fighting breeds who are dog-aggressive are simply predatory toward other dogs, many of whom are smaller, lighter or less willing (ie, try to escape or run away before there's any physical contact).
I've also read those old stories where a pit bull was a child's best friend, and men could break up fights with their bare hands. Another aspect of many of those stories was how the champion fighting dog hero wouldn't bother with an 'inferior' dog, turning away in noble disgust before a poodle's yaps. That is demonstrably not the case anymore, which makes me think that of all the damage done to all the breeds since the start of the 20th century, even the crippling of the English Bulldog and the dumbing-down of the Rough Collie doesn't equal the changes wrought on the pits.