By the time a 911 operator gets done asking the questions so you answer them the way they want them answered the guy is already beat to hell. Compel someone to call while you, if you choose to, engage. The victim could be dead before the responding officer gets the call.
Maybe I'm not very smart, but I can't see myself standing around while another human being is being beaten. This guy survived, but freak things happen and the beating this guy survived may have been enough to kill someone else. I'd have a hard time living with myself if I didn't at least try to help.