The fact that those tweets exist and are given a platform on which they can spread the influence is what's harmful.
The words themselves can be hurtful, of course, but they're not as harmful as the power they have when they are tweeted out for the whole world to see. They are given value by being allowed on twitter. They are viewed as valid opinions that new people can adopt and spread. This is why I think twitter is doing the right thing by actually banning such accounts, and why simply giving an easier way to blanket "block" those accounts from your personal view might not be enough.
The words themselves can be hurtful, of course, but they're not as harmful as the power they have when they are tweeted out for the whole world to see. They are given value by being allowed on twitter. They are viewed as valid opinions that new people can adopt and spread. This is why I think twitter is doing the right thing by actually banning such accounts, and why simply giving an easier way to blanket "block" those accounts from your personal view might not be enough.