Even if we're confining matters to verbal attacks, I think there are some points you're missing. If (for example) someone uses a racial epithet in speaking to someone else, do you really feel that it's inappropriate for anyone else to say, "Hey, that ain't right"? Because that's the kind of thing we're talking about here.
I think even in that situation the best thing is for the person who it's directed at to say "hey, that ain't right." Simply because that's the most powerful way to deal with it. Anyone else is going to be treated like an outsider by the "attacker". I see it happening all the time on different forums. Other people getting involved inevitably means the argument growing and discussions getting derailed. And in the end pretty much no one changes their behaviour.
By the way I'm not saying one can't say something supportive to the target of the verbal attack. I know in my own case if someone would attack me on a discussion forum, I'd appreciate it much more if people would tell me that they understood my point of view, instead of getting into an argument with the other person (who by that time I probably would think is an idiot anyway