Diet drinks are actually not terrible if you're trying to cut excess sugar from your diet, yet still need something sweet from time to time to ease the cravings. There have been studies conducted where people who were allowed to drink diet soda as part of a weight loss program had
better outcomes than those who weren't, not because Coke Zero has magical fat burning properties, but because the people in the artificial sweetener group found it easier to stick to the program.
It's sort of like how people use nicotine strips to quit smoking -- obviously it's better to just
stop drinking soda, but that's often easier said than done. Personally, I only ever drink diet soda because drinks made with real sugar make me terribly queasy and I think the diet options often taste better anyway.