Banning Pit Bulls would be like banning cars because people get killed in car accidents! Who's responsible, the car or the driver/manufacturer? Any car can be deadly in the wrong hands or if built with defective parts. Same thing with dogs... Any dog. Pit Bulls are no more responsible for the way they are bred, raised and trained, than cars are responsible for the way they are designed, built and driven.
Simply put, the best argument against breed bans is that they are costly and ineffective. Breed bans are often a knee-jerk reaction from politicians who want to say they are "doing something", after a highly publicized dog attack (of any breed). This is a useless exercise. What kind of message are we telling abusive and irresponsible individuals when legislation makes the dogs pay the price for their action?
Criminals habitually break laws, so having an "illegal breed" may indeed be attractive to undesirable individuals and entice them to breed and sell more "illegal dogs". If their dog is confiscated and killed, they don't care. They will just get another one because BSL punishes the dog, not the owner.
On the other hand, law abiding and responsible owners, whose dogs love people and have never done anything wrong, can see their homes invaded, often without a search warrant, and their beloved family member dragged away (in front of their children) to be killed. Not because the dog was unstable or mean, but simply because of its breed. Meanwhile, the owners of truly dangerous dogs (of any breed) escape punishment because their breed is not targeted by legislation and therefore believed "safe".
A 10 Lbs Pomeranian killed a baby a few years ago...