Why the importance on the thesis statement?

Yesterday in class we had a very long discussion on one of the students papers, concentrating almost entirely on her thesis statement. Now I know a good thesis can make all the difference on a paper but should it really take more priority than the paper as a whole? Atleast that is the impression I was getting from our class discussion. I believe that a thesis should clarify the argument/stance you are trying to make in your paper, but I don't believe that it should make or break a paper, especially if its the case that you have good supporting paragraphs and arguments that clearly define what you are getting at.