When you falsely, misleadingly and/or dishonestly attack someone, I think it can create the impression that there aren't legitimate criticisms to be made against said person. Or that an honest description of them wouldn't be enough to create a bad impression. That's partly why stuff like this bothers me (aside from the fact that it's also bad in principle). Richard Spencer is a bad hombre, no question. He seems to be an anti-semitic, white supremacist. Why is that not bad enough?
When you're defending or advocating assaulting people for what they've said and who they are, I'd say that makes it doubly important to not misrepresent what they've said and who they are.