Having only the details you posted, here's my take on it.
1. Once you made it to your car, it sounds like the belligerents weren't interested in you. If that is the case, in your shoes, I would have stayed at the car, perhaps even inside it, weapon ready but inconspciuous, gotten on the cell phone to call the cops, and watched from a distance to be a witness. You can't be sure who started the fight, who, if anyone was in the rights, so intervening is probably a good way to get burned and wind up in the paddy wagon with the rest of the bunch.
2. I certainly wouldn't have touched that knife!
My opinion is based ont he fact that I have no desire whatever to be a cop or stick my nose into other peoples' disputes. If someone I know was being threatened with a broken bottle, it becomes a different story.
I agree with you on the general wet-diaper attitude of many people, though.
My 2 cents, for what it's worth.
EDIT: I suppose if it was 6 guys piling on one guy to beat him sensless or stab him to death it might be pretty tough to stay out of it. But I would try to stay out if I could.