Can you actually change a person's mind by writing a carefully constructed argument which calmly and clearly shows why the person should change their mind in the first place? It seems to me that people always preach to the converted. You can write or argue till the emigrated South Africans come home, but you will find agreement only with those people who shared your sentiments in the first place.
I have been reading posts on some groups on facebook, where the same arguments happen again and again. Affirmative action for instance. Is it good or bad, racist or merely neccesary? I will not enter into this debate right now, but a person who is for affirmative action will convince few people against it to change their minds. I have never seen a post on a group along the lines of "wow, I see what you are saying, I convert to your thinking." Generally people read what they want, hear what they are programmed to hear, and seize up at the first hint of a challenge of their beliefs.
So why do people keep writing? Well, it is comforting to have people agree with your opinion, it lets you know that you are not the lone dot on the "previously lit" landscape that thinks a certain way. And it is good to have all views, all sides to a story out there in the public domain. It is essential to keep people on their toes. If you assume your view is dominant, or the only one out there, you tend to lose your ability to think critically. There needs to be diversity and opposition.
Writers will keep writing, and trying to change peoples views for the better, but who on earth can claim that they can define what "for the better" is?