The ball makes you work a little to retain your balance and sit upright? Is that the idea? It sounds like an interesting theory. I'm not sure how well it would work in practice. It seems like the frame and the backrest defeat the purpose at least partially. I have spent one or two hours at a time sitting on a bare exercise ball while watching movies, and it was neither unpleasant nor heavenly.
If you suffer from back pain, as I did some years ago, I think the second best thing you can do is to dispense with a chair and work standing up. The very best thing you can do is to strengthen your back through regular exercise.