Well, 2 reasons, maybe
Ordinary Coke contains an absolutely horrifying amount of sugar. That's a lot of calories. Even if you're eating a burger, there's surey no harm in missing the bajillion ounces of pure corn syrup you'd be otherwise having on top of it. Especially when you're telling people to "do something about it". That's doing something, if not a lot.

Also, Diet Coke tastes different. Nicer, in my opinion. As well as the better taste, all the sugar in ordinary Coke gives me a headache. It's possible to order a Diet Coke because you like it better.
(SusK, Fri 2 May 2008, 17:49, Reply)