If you are being helpful and the person you helped needs you to testify or anything else then you should be responsible to help them more. If you make matters worse instead of calling the proper people then you should be responsible for the damages that were caused because you intervened. Either way you have a responsibility since you stepped into the situation by your own free will.
We have a whole system already in place to maintain law and order in this country. It is a system designed to deter vigilante justice, and bystander intervention should be no different. This is not to say that bystanders should not try to gather as much information as possible, in order to assist law enforcement, but the line to distinguish helpful intervention from committing a crime oneself is too hard to define. In that sense, it is probably safer and more efficient to avoid intervention altogether and leave it to the professionals, rather than potentially initiate a whole new situation.
No, I do not think that bystanders who witness a fight or other trouble should legally be responsible for jumping in and stopping trouble. They should not have to put their life in danger to help out someone else if they do not want to jump in and help them.
No, individual bystanders should not be legally compelled to act if they observe an illegal or dangerous event. While some would like to punish those who don't meet their own standards of morality, it is important to understand that individual freedom is the means by which they were able to develop that very ethical code. The same individual freedom should be afforded to everyone, even if they don't choose to be altruistic.
If it becomes illegal to not assist someone in trouble or someone who is injured it will create a much bigger problem. Last year I was in the vicinity of a car striking a young girl in the street. I did not do anything to help because I have no medical training. Someone else had already called the paramedics and luckily we had an EMT and a nurse in the crowd who both stepped up to help. If people are afraid of getting in trouble they will try to help out and will cause more harm than good.