Being overweight has significant associations with type II diabetes, all cancers except esophageal (female), pancreatic and prostate cancer, all cardiovascular diseases (except congestive heart failure), asthma, gallbladder disease, osteoarthritis and chronic back pain. These are the results found when 89 studies were looked at and the data compared.
Despite this evidence, there are people who will argue that being a healthy weight is not important. There are books and articles written saying that it's not a health issue, but rather one of moral judgment. There are some people, who though obese, are not showing signs of any of the above illnesses. They are classified as MNBO. These people have lower mortality than healthy weight, but under-active people. The number of people who are MNBO is small and they may be in that category because they're in a weight loss program at the time of the testing.
Not everyone who eats healthfully and works out regularly will achieve an "ideal" weight. That does not mean that people should give up on achieving a BMI in the healthy range. You should place your daily focus on eating well and being active; that will lead you in the direction of a healthy BMI and a better life.