In my opinion any and every dog has the potential to bite and harm, should they decide to. This tends to happen far too often as many owners are completely ignorant of what their dog is capable of. A bit of knowledge and common sense goes a long way...
I may be biased in my opinion, as I am a professional dog trainer and behaviourist, but I believe that it is the responsibility of every dog owner to at least have a basic knowledge of the breed of dog, and the associated characteristics thereof, that they intend on taking into their care. If the time is taken to find out more about their dog, then they can understand where potential 'problems' could arise, and deal with these in the appropriate way before they develop.
For example, Staffies do have a tendency to be very strong-willed, boisterous and aggressive, if the correct training, socialisation and exercise is not provided. I therefore believe that anyone thinking of taking on a Staffie should be more than aware of this, and actively work with their dog from day one with these 'issues' being their main focus, to counter such traits from ever developing.
It is easy to think Staffies are horrible dogs who are all dangerous and aggressive, when there are so many examples of them standing on their back feet, straining at a lead, snarling and barking at anyone or anything that comes near them, whilst looking like if they were given half a chance, they would rip whatever had caught their attention to pieces.
However, I have experienced Staffies owned by responsible owners, who have taken the time and effort to produce well trained, balanced, well exercised and stimulated pets, who are a joy to own. One of the most affectionate dogs I know is a Staffie who loves nothing more than to curl up on my lap as soon as I see her, then fall into a contented sleep and snore very loudly!
Her owner was more than aware of what he was taking on when he got her as a pup. He knew that she had the potential to be very strong, boisterous and, in theory, dangerous. We therefore spent plenty of time working with her to ensure that she was as obedient as possible to commands, especially recall and dropping anything in her mouth when told (after all, should she start playing with a child/ animal and get carried away, with those jaws, she could do horrendous damage! Thus making sure she dropped something, no matter what it was, as soon as told, we felt was very important) in an effort to limit nasty things from happening. She was very well socialised with many different people and dogs, and is perfectly happy and relaxed around everyone.
Further more, he had the intelligence not to put this dog, or others in situations where something horrid might happen. He would never dream of leaving his Staffie unattended around a small child, no matter how sweet natured his dog is. All it takes is for his dog to think a child wants to play, or get excited/ unnerved by the squealing noises and unpredictable movements which kids are known to make, get the wrong idea, hurt the child, and then face a death penalty for her mistake!
I managed to end up with a Rhodesian Ridgeback x GSD pup a few years back. Knowing nothing about this pup, I had no idea of what her temperament was going to be like, or what size she would end up. My father bred GSDs for many years, so I knew how wonderful they could be, but again they can also be aggressive dependent upon their training/purpose and the situation.
All I knew a bit about Ridgebacks was that they were quite big and originally bred for hunting lions; I decided to think the worst!!! I assume that I could end up with a massive dog, with strong hunting/hurding instincts!!! I therefore felt it very important that she should be exceptionally well trained, as her size and strength could be a problem. One of the first things she learnt was not to jump up, as I thought she could either terrify or flatten someone!!
It turns out that she is actually quite small for both breeds, calm, incredibly intelligent and wonderfully attentive. My partner actually believes she has been 'over-trained', as she will simply not do anything unless I tell her she can, and he feels she has lost some of her personality. This may be the case, but I feel safer knowing that I have as much control over her, as anyone can possibly have over another creature with their own personality.
I put the above in, as although I trust her as much as it is possible to trust an animal, I know that in a given situation, she is still a dog, capable of thinking for herself, with an instinct which is very much present, no matter how much training is given, and how soppy she usually is. This manifested itself when I left her in the garden with some chickens and a gosling once. After 5 mins, I realised I had left her outside, went out, only to find she had killed two chickens and the gosling. She instantly knew she had done wrong, and fell into a submission position.
Lesson learned...She is still a dog, and therefore her behaviour cannot be 100% assured.
More people need to be aware of this! I couldn't guarantee that my partner, who is probably the most chilled out person I know, wouldn't 'blow a fuse' given the right/ wrong situation, so, if I cannot make such a guarantee for him, why would I be arrogant enough to do it for an animal, which is what so many owners seem to do?!?!?!
As for Bull Terrier types etc 'always' being the ones to attack, this is BS. It is this type that are always 'reported' as being responsible as it is more sensational to report such things. I'm sure if the media were to report every single dog-related incident, it would be full of small breed dogs being anti-social.
It is almost a shame that these aren't all reported, as it might actually make people more aware of the fact that small breeds carry the same potential to injure as large breeds, and more might be done to curb/stop such behaviour.
Sorry, that is my two cents (and a few dollars more!) worth added to the thread, but is does annoy me when a blanket generalisation is given.