Remember too that any carbohydrate you eat that gets converted to glucose and then glycogen, but can't be stored in the liver or muscles (VERY limited storage capacity), gets converted to triglycerides which are then stored as body fat. Even a low fat diet can still make you fat if you have too many calories from carbs.
Your body doesn't allow glucose to just slosh around in your bloodstream - it has to move it out (insulin's function). The quicker the food gets converted to glucose (higher GI) the less likely it is that you are going to burn ALL those calories from that food in the time that your body's going to allow the glucose to be in your blood...
Slow release of glucose means you use up the glycogen before it gets stored as fat. (Hence low-GI stuff being better for us LCers)

Cheers!
Steven