The thing about making something illegal or banned, is it only keeps law abiding citizens from owning/possessing that current thing.
So will dog fighters keep fighting pit bulls if they are banned? yes, they will as dog fighting is illegal to begin with and they have no respect for the law.
Will drug dealers put them in abandonbed houses to keep people out stille? yes because drug dealing is illigal and they have no respect for the law.
Will it keep a burglar or someone trying to cause harm to a family and/or their possessions? No, unless that family fights dogs or deals drugs.
Good dogs belong to good owners. If the owner is a bad owner they will have a bad dog regardless of breed. Pitbulls are very loyal dogs that love and will do anything for their owners. Unlike some other breeds.
Temperament:
Courageous, loyal, full of energy; Should be socialized early on with children and other animals.
From
http://www.bulldogbreeds.com/americanpitbullterrier.html