You might be right about people who believe in crazy conspiracy theories. Usually, if anybody gets really fanatical about some cause, there's nothing you can do to change their mind.

However, by being aggressive and dismissive, we are insulting people that are on the fence, as well as the opposition. They will feel stupid and will see the other side as being more reasonable and accommodating. That is why I say that this kind of comment is not useful. It's actively pushing people in the center towards the other side.

EDIT: Accidentally posted without finishing typing the comment.