As a police officer of nearly 20 years I would certainly hope that a good citizen filming me struggling to take a suspect into custody would lend a hand.
That is a truly brave man in the clip you showed. There was an article on the BBC News asking the question of if people should help instead of filming. The experts that were interviewed said filming does help but the most important thing is for people to gauge their safety.
I think it depends on the situation. I'm likely to help if I can... but... who can know what they will do until they are in that situation.
I see the corruption, I see the good intentions gone wrong, and then I have worked in places where I was grateful for police presence. It's a thankless job these days.