If I was not being paid to keep the peace, hay-ull no! First of all, if one of them does have a blade or is armed in some way, you can get seriously maimed, maybe even permanently, and if you do, then what? Lawsuit? Sue them for what? They're homeless!
Secondly, I don't know what the laws are like where you are, but should you have to apply any kind of restraining force to, let's say, the face-stomper you better hope that enough friendly witnesses stick around to corroborate your version of events when the cops show up.
Granted, most municipalities here have good-samaritan laws that protect people who are seeking to protect others from "death or great bodily harm", but the definitions of such are open to a lot of interperetation. Should you have to clobber the face-stomper to get her off of the stompee, you can at the very least expect to be held and questioned for several hours.
At worst, you may get arrested and have to prove your innocence in court (good luck trying to subpoena witnesses in a large, hectic crowd like you described). Worse still, things may not go your way, and you will have a battery charge on your permanent criminal record!
I think this is why people don't get involved when domestic disputes or whatever go down in public; not because people are uncaring, but because they are scared to death they are going to get sued and/or arrested for taking action.