In the morning, your body has gone 8+ hours since eating or drinking anything. Your blood sugar levels are lower at this point, and your body doesn't have adequate fuel to workout optimally. Usually, experts recommend eating something--even if it's just a small snack--within 2 hours before working out. When your body doesn't have proper fuel in it, many problems can result, the lesser being that your workout performance suffers, and the greater being something like passing out during exercise.
However, every body is different. Some people can workout on an empty stomach with no problems, while others would end up very sick and feel the negative effects of it.
Also, I think there might have been a bit of confusion here about metabolic rates in the morning. Eating breakfast in the morning has a positive effect on your metabolism, but exercising on an empty stomach does not. Some people say that it will burn fat stores, but overall, the number of calories your burn during a workout (regardless of where they come from) is much more important. Plus, fat burns in the carbohydrate flame. This means that exercising without eating (such as after "fasting" during sleep) your body does not burn fat efficiently, or sometimes at all.