Bananas are low on the Glycemic Index because they are high in fructose (as are all fruits), not glucose. Your body produces a lot of insulin in response to glucose, not fructose, though they are both monosaccharides. You can store glucose as glycogen, but you cannot store fructose as carbohydrate, so excess fructose is converted into fatty acids. This is why HFCS is so bad. People essentially eat way more calories than they need at the moment, and since these calories can't be stored as anything but fat, they ARE stored as fat and you feel hungry again soon. You also get some hyperglycemia problems if you eat a lot of it, which can lead to Type II diabetes, and your liver gets kind of f***ed up.
Really, I wouldn't worry about it too much. Fruits are mostly just sugar and water. They are also an essential part of the diet, but you also shouldn't eat a ton of them either for optimum nutrition (you generally aren't going to get any "extra" nutrition from eating more than a few servings a day). In general, try to eat some fruit each day, and try to limit other sources of sugar in your diet, especially HFCS.