It absolutely is. Making nazis look like a bunch of fools is the solution to people wanting to be nazis.
The breaking of windows isn't all that useful if we are looking at the big picture. It is pointless. Fair enough.
Setting legalities aside, the specific event of Spencer getting punched in the face is a different story. The effect was more mockery than thuggery. Whatever the intention of the guy doing the hitting, the effect was more comic than vicious.
The value of Spencer getting punched in the face wasn't the violence as much as the embarrassment and humiliation of a movement that worships strength and innate superiority. Had Spencer been hit in the face with a pipe and wound up in the hospital with serious injuries, then it might have had propaganda value. Same if the whole black bloc had beaten him to a pulp. That didn't happen. This was more like being hit in the face with a pie. That would also be illegal, but the added comic element makes it more clearly intended as a statement of contempt than intentional harm.
Those wringing their hands over this making the nazis look sympathetic are largely creating the problem they are worried about. Rank and file nazis become nazis because they are frustrated with their lives and are looking for a way to feel superior. Having the country laugh at you when someone shows you physical disrespect is not good recruitment material for these groups. It speaks to them in the language they most understand.