The average person can lose 15 pounds over year by replacing regular soda with diet soda, not 15 pounds per year. Yes, that might not make a big difference on somebody consuming 8000 calories, 200 grams of fat with a weight of 500 pounds. However, for somebody 20 pounds over weight, it can make a big difference.
For instance, somebody drinking a litre of coke a day is taking in what 750 calories or so? If you replace that with diet coke you just stripped out 750 calories a day from your diet.