Colon Cleansing?

My friend and I are having a debate about colon cleansing. He says that everyone should have it done and it’s necessary for optimal health. I say that it’s not necessary, but only optional. I think that the authors of those books and the creators of these colon cleansing programs are just trying to find a way to take advantage of and make money off of people who don’t know enough about the human body and how it works. What are the pros and cons of a colon cleansing? Is it really necessary?