I've been thinking about it, and I think it's important to note that there are at least four sides here. It's not only Antifa and Nazis/KKK/bigots/white supremacists/fascists. Two other sides are bystanders, and police.
The police are a separate side. And that's what necessitates Antifa in general, and the black bloc active-force branch of Antifa in specific.
Because the alliances break down in different ways in different places. Mostly, the police are primarily allied with the bystanders. And, mostly, the bystanders are allied with "whomever is least disruptive."
In Boston, "bystanders" were allied primarily against Nazis, and that meant that they were incidentally, and loosely, allied with Antifa, and that meant that the police were primarily allied with bystanders and therefore more on the side of Antifa than Nazis -- but that doesn't mean they were on the side of Antifa.
The GOOD news is that, in Boston, the police are allied primarily with "bystanders". And, when we were sitting next to a black bloc guy on the train into the demonstration (nice guy, named Tim, he was going through a lot of trouble in his life, but having the direction of working for social justice was giving him something to hold onto -- he'd been clean from heroin for two months, which is longer than he'd ever done before, and he looked damn good for two months clean. I should have got his contact info -- I really liked him, and should have tried to keep him as a friend), I mentioned that I was hoping that he'd give the police the benefit of the doubt, because I expected them to be more on the good guy side than in, say, Phoenix or Charlotte. And the black bloc folks mostly did, and the police mostly were.
In places where the police are honest and decent, they ally with the bystanders. In places where the police are corrupt, they form their own separate side.
And in places where the police are honest and the bystanders are generally not assholes, we, frankly, don't strictly speaking NEED Antifa, at least not as much as in other places. It's USEFUL to have an Antifa presence in Boston, but, for now, if we didn't, we wouldn't be screwed.
In general, violence is a tool, one that we hope never to use, but, if you lack the capacity for it, then those sides that DO have the capacity for it can roll right over everyone else. The police are the side with the greatest capacity for violence, and, in places where they are on the side of the bystanders, the weight of the power of force is on the side of the status quo.
And that's not great, but it could be worse.
Nazis/Fascists have no compunction of using force. So we can always assume that white supremacists have some degree of option-of-force in their pocket.
People who are "against Antifa" or, more accurately, against black bloc tactics, are saying that they want the good guys to unilaterally disarm. If they really thought about it, they'd want the Nazis to unilaterally disarm, too, but it seems like they're working under the assumption that the Nazis ARE disarmed, which is, y'know, not only false but actually stupidly false.
If the police were reliably good guys, this would be reasonable. But in Charlotte, the police allowed Nazis to carry weapons and even fire them, and didn't act until black bloc Antifascists responded. The police in some places are actively in the bad guy camp.
If the bad guys have the capacity for force, and the supposedly-neutral guys who have the greatest capacity for force can't be reliably assumed to be neutral, then it is necessary for the good guys to have the capacity for force.
And that's what the black bloc is. In places where the police do their job, they're not needed. But if the bad guys have the capacity for violence, and the good guys don't, then the police have a reason to side with the bad guys, because the goal of the bystanders is to keep everything quiet, and letting one side roll right over the other is the easiest way to keep things quiet.
Having the black bloc means that the police have to remain honest.
And, let's face it -- that's exactly the theory behind the Second Amendment. Even if the black bloc doesn't use firearms, the Second Amendment is a recognition that the Founders were aware that the capacity of the people to use force is a necessary condition to keep those who have official sanction to use force honest. The existence of the black bloc makes it less likely for them to be necessary.