Congrats on preparing to do the flush, it will give your body what it needs to rest and restore. You are right Prune juice alone can keep many people's bowel's moving. However some people have very sluggish and constipated bowels that require some additional assistance, thus the Bowel Cleanse recommendation. I would also be interested in knowing what first hand experience the person who wrote about senna tea has or if it is all just "scientific" book knowledge. While Schulze's cleanse does not include senna tea, senna is one of the ingredients in his formula. Interestingly it has been used for centuries to assist in the jump starting and toning of the bowel muscle throughout many ancient and modern societies. It's role is to give the bowel the nutrition it needs to operate on its own.
From first hand experience I can tell you it is excellent at this and unlike coffee, enemas, and other stimulus it is not addicting, toxic, nor does it take the place of the bowels work. If I were you I'd carefully consider the information you read.