Let's ask the practical question.
Assuming that some points of view should be "denied" the use of a platform, who decides?
Twitter right now is censoring based on some model of public opinion. They are to make everyone happy and yet no one is happy. I think Facebook just builds user models, shielding each person from things they know you don't want to see. I am not happy with either of these models... it just pushes us farther into corners. Of course, there will always be a platform for offensive views unless the government steps in... and right now we have a Republican Congress and Trump (be careful what you wish for).
There are a significant number of people who are offended by CAIR. There are others offended by La Raza. There are others who are offended by Planned Parenthood.
Do we vote on which of these viewpoints gets to use a platform? Is it different by the region of the country? Do Public Universities really get to blacklist speakers; I bet the blacklist of the University of Texas would be quite different than that of the University of Massachusetts.
These decisions are necessarily subjective and our country is deeply divided. Sure, Richard Spencer is an easy case (for most Americans). But how do we draw the line?