It really depends on the situation. If it's one on one fight, clearly unarmed, people should intervene.
Like the recent cyclist vs motorist road rage incident, that was a perfectly good example. Driver Attacks Cyclist Road Rage Video
Someone happened to film it, and 3 guys came in to break it up.
If it was a whole mob fight, there's really not much you can do.
Maybe there's a hero in all of us, but realistically, for someone you don't know,
are you really going to sacrifice yourself to jump in and most likely get jumped as well?
Unless you're "fictional version Ip Man" who can take on 10 guys, chances are you won't be helping the situation.
And in hindsight, it's very easy to say someone should have stepped in,
because now we know it was severe enough that someone died.