You could look at it from the other direction. Once you agree that outright fascism is not to be given a platform, fascism lite (and I think there's some evidence that 'alt-right' is a sort of rebranding of modern white supremacists, white nationalists, and neo Nazi types) should maybe also not be given a platform.
Death by a thousand cuts. Today, calls to institutionalize trans people, undo the civil rights act, or put gays through electroshock therapy. Tomorrow, expulsion of the Jews?
It's a 1000 cuts thing. Yesterday the nazis, today a breitbart editor, tomorrow?