It's relative. Compared to plain old water, you're correct. But when one is used to drinking Coke and beer, a no calorie alternative is better.
True. When I was a kid, all my family drank was Coke. We bought the 32oz six packs of returnable bottles, and we went through them fast. I hated the taste of water, iced tea and just about anything else.
When I was in my 20s, I realized that all the caffeine and sugar was making me alternately buzzed and exhausted. I tried diet drinks, but they tasted horrible. Then I tried Ozarka bottled spring water (a local brand), and I was hooked. I used to have a 5 gallon Ozarka water cooler in my house, with weekly deliveries.
Then Nestle bought Ozarka, and the flavor changed. It started to taste dull. I searched about again for something to drink, and found a Pur filter pitcher that sits in the refrigerator produced the best tasting water. The water tastes great, I don't have to lug bottles from the store to my house, and I don't have to pay through the nose for water. Now that's what I drink. In the morning, I have an N.O. Brew iced coffee, but the rest of the day it's filtered water. I'll drink iced tea at a restaurant if it's decent. I see no need for diet drinks. They taste better than they did in the 70s, but not nearly good enough to actually drink.