When I was practicing law, I handled a number of dog-bite cases, and it was usually true that a "bad dog" was the responsibilty of the humans: vicious dogs were neglected, if not actually abused, and poorly trained if trained at all--usually. There are way too many yahoos and idiots who have to have a big tough "mean" dog as part of their panoply of phallus substitutes, just like they drive a big tough truck they don't need, and buy big loud guns they don't know how to shoot or maintain. Unfortunately, every so often there is the case where a heretofore friendly dog mauls someone without apparent provocation. And I'm well aware of the statistics quoted by large dog owners that small dogs bite more often. However, if a chihuahua or a yorkie nips your ankle, it's shruggable. If you get bitten by a dog with inch-long fangs and jaws capable of exceeding 400 pounds per square inch of biting pressure, it's a serious matter. I'm always rather bemused by the fact that people who would not think of having a pistol in the house, even if unloaded and locked in a safe, thnk nothing of having children play with the family dog unsupervised, even though you can't "unload" the dog, unlike the pistol the dog is capable of going off by itself, and can pick its own target for reasons not necessarily explicable to humans.
My suggestion would be that powerful dogs, such as pit bulls, Rottwielers, Dobermans, German Shepherds, et al., should be covered by some fairly stringent regulations. Breeders should be inspected and required to account for all their animals so that they don't "disappear" into fighting dens. Buyers should have to show proof of having completed an accredited course on dog training, handling, and care.
I realize that this would be onerous on the breeding industry, but it's better than being outlawed entirely: and, frankly, I don't think the average person needs to have a big dog any more than they need a pony: there are plenty of smaller, clever, friendly breeds that make excellent companions for any need. (Actually, my reccomendation for most people is to go rescue a cute clever mutt from the pound. I think the amounts even modest-income people are willing to spend on a purebred dog or cat are obscene--but that's a different rant--).