Seriously why if it is something you enjoy? Zero carbs Zero Calories. I drink lots of soft drinks as I hate coffee & fruit juices hurt my stomach and are only 10% pure plus are loaded with sugars. I drink soft drinks with Splenda {Suclarose} for the better taste. Actually I like Splenda sweetened cokes better than regular ones now. Zero calories Zero Carbs there is no way it can cause weight gain.
Actually on fitness sites diet soda's have been known to cause weight gain for years..these are people who write down everything they eat and drink and when they drink a lot of diet soda their weight goes back up. I know it does for me if I drink 2-3 a day and I keep track of every calorie even my supps I take.
I can't remember the exact science but heavy aspartame exposure directly contributes to increased blood glucose levels which leads to increased fat.