It is not a waste of money if your replacement water has high organic compounds to begin with. When you have water like that, you can do all the water changes you want and you will only suceed in removing the additional og's created by the artificial envioronment. More people are in this situation than realize it. It is also why people can see clearer water than without the carbon.
If you do not have that issue, then don't use it and save the money. I have lived in 8 states since starting this hobby, and always had clearer water and better conditions with the use of carbon.