MBA provides you an opportunity to systematically acquire the various basic techniques and toolkits that you will need to make business decisions. It also puts you in an environment where you can meet other potentially successful business leaders. You can always learn those on the job and meet those people through other means. School is not going to magically equip you with the good "business sense" or the path to success. It does, however, allow you access to a wealth of good (and bad) examples to draw on when you do need to make important decisions.
Days ago, Dean of HBS, Jay Light, gave a presentation in my shool. As for what MBAs can learn in school, he said,"Twenty years after HBS MBAs graduated from school, they will not talk about the specific technique they learnt in classroom. What they will talk is the learning process itself, the way how they handled questions, the judgment and insight of leadership they got and a whole new mentality they shaped along the process”.