You Ask, I Answer
Ah, the 'dirtbag left'. An echo of the days when a dumping ground for libertarians on Something Awful became a 700+ active user forum for radical-left discourse to the point that the Secret Service seized the servers. In the aftermath, I discovered the term 'irony poisoning', which has become a foundational term for my worldview.
My knowledge of the dirtbag left itself extended as far as some controversy over a usage of the phrase 'bend the knee' potentially having sexual undertones. It seemed like an epitaph of its legitimacy and influence as a movement.
As far as the political efficiency of punching Nazis goes, I think you are slightly underrating the value of antifa having something to do. It seems reasonable to suspect that most of its members are of a class of un- or underemployment and desire to do something flashy with their youthful energy. Especially given how well-regimented they have been, as you note, I suspect that some of them would be ~*hurting the left*~ if they didn't have their Nazi-punching collective.