The biotech's shares dropped following its latest quarterly update, but that shouldn't matter to long-term investors.There are several textbook arguments against monopolies. According to one of them, companies that control a market can lower the quality of their products with impunity.However, Vertex Pharmaceuticals (VRTX 0.91%), a drugmaker with a monopoly in the cystic fibrosis (CF) market, hasn't done that -- quite the contrary. Since it launched its first CF product some 12 years ago, the biotech hasn't ...