These are valid points, but for me the biggest difference between Art and Activism is Audience. I couldn’t care less what people want to include in their definition of ‘Art’, because it doesn’t mean that I have to think that it’s good art. Rather, I’m more interested in why someone wants their work to be considered art. When activism is called art, it changes who talks about it, who reads about it, who thinks about it. It also creates expectations for aesthetic creativity as well as a dialogue with the history of art. This has advantages and disadvantages, of course, but it certainly changes the dynamic of what can be accomplished.
What I’m getting at (slowly, and in a round-about way, I know) is that the defining characteristic of all Art is that it creates new experience. Even though an artist such as Matisse was not a political artist, he was attempting to facilitate new visual experience by creating new visual images. This new visual experience can then lead to changes in society (the goal of political art) whether intended by the artist or not. Perhaps the main difference between ‘art for art’s sake’ and ‘art with a purpose’ is that ‘art with a purpose’ takes a more active role in what it affects.
The first paragraph makes a point similar to one I was trying to make when I worked with Peter Fend as part of my degree show years ago. The above is much better expressed and the corollary in the second paragraph quoted is a hopeful proposition. I can’t remember what I concluded from my work with Peter, I think I was more or less throwing the ball back into his court.