Yeah, it turns out that punching someone in the face is a good way to give them a lot of free publicity. A video of Spencer giving a speech at a neo-Nazi conference got 129,000 views. The video of him getting punched is at 2.4 million. I'm sure most of those views came from people who just wanted to see a jackass get smacked around, but how many people do you think saw a man who was trying to calmly express his views, only to get attacked for having an opinion?
"Oh, but who could possibly believe Spencer's nonsense?" you ask. "He wouldn't give a shit if a black man was beaten to death in the streets tomorrow, and you only have to listen to him for a few minutes to realize that he's full of shit!" Sure, but this isn't about you. Whether you're a hardcore socialist or a moderate Republican, Spencer doesn't care about you. You're already a lost cause to him, an adult who's been sucked in by the evils of the mainstream.
Spencer wants impressionable kids. Here's an article about his efforts to recruit college students. "I think you do need to get them while they are young." Or listen to the journalists who have gone to alt-right conferences and found a lot of young, angry white kids. White kids who are worried because they can't find a job that pays okay, who are lonely because they're no good with women. Here's a former white supremacist talking about how he was angry all the time when he was a kid -- at his family, at school, at life. These are not happy people.
Those white kids hear two narratives -- one from people like Spencer, who tell them that white kids are the vanguard of civilized society, and one from people who cheer Spencer being punched, then complain that white men are ruining the country. "But if you put the idea of the patriarchy in the proper historical and social context, they'll see that actually..." No. Stop. Angry, lonely, scared teenagers aren't going to appreciate a nuanced lecture on how centuries of male dominance has produced a system that oppresses them just as much as everyone else. They just see society kicking them while they're already down (it doesn't matter if you think they're "down" or not ... it matters if they do), then Spencer offering them sympathy and a hand up.