what's wrong with 'fake' sugars?

Hi guys,

Sorry I need some convincing...or more information to help me understand better. :h11)

Why exactly is 'fake' sugar bad..I mean, if it is not sugar, but instead it is splenda, or the other replacements, isn't this favourable as it would not evoke the same response in the body as sugar? If we were to have diet coke for example, wouldn't this help with the cravings?