What is your opinion about waterproofing a chimney?

Ask whomever is trying to sell your this service what the benefits of doing it will be and what the consequences of not doing it will be. Don't let him give you any "it might"'s. Then you can make an informed decision about whether to do it or not. My thoughts are other then making sure you don't have any water coming into the house from the chimney, why would you want/need to do it.