Moderate workouts burn more fat and other crazy theories

I'm no exercise physiologist, but I usually put a lot of thought into things that relate to my body. I've heard many theories reiterated recently about exercise and nutrition and some things just don't add up for me. First theory: Keeping your heart rate between 120 and 150 is the best way to burn fat. While this may be inherently correct, I think it's deceptive.

Workinig out at a moderate intensity may cause your body to burn more fat percentage than glucose percentage, but common sense tells me that if you workout harder you'll eventually burn your glucose stores and then move right on to burning fat. So, in essence, you're probably burning the same amount of fat or more as a moderate workout but you're burning more glucose, plus your metabolism is going to be moving faster as the day progresses. Once your body runs out of glucose it's going to move on to something else. It HAS to. I'm sure it burns some muscle in addition to fat, but I haven't seen any studies that show your body will burn more muscle if you work out at a higher intensity. And adding weight training into the mix ensures that you'll keep your muscles and therefore your metabolism, which means more fat burning.

Now I can understand that working out at TOO high of an intensity can cause problems. But I don't buy having to cut back how intense you work (and for a longer time period) in the name of burning only fat stores. You'll burn it now or you'll burn it later.