I have had arguments with myself on effects of consuming "diet" foods. In my quest for a sub 10% bodyfat, it is easy for me to lean towards the various sugar free alternatives of food and soda.
I know many people will disagree with me on this one, but I find these man-altered foods to be practically useless. So many people believe that if they make a dessert out of fat-free, sugar-free pudding, then it is "healthier."
It may contain less calories, but what the hell is it made from? Have you ever read the ingredients of some of that stuff? One common ingredient in "sugar-free" foods is maltodextrin. Maltodextrin is a glucose polymer, and while it's technically not a sugar – it's still a sugar! So while your Jell-O may be "sugar-free," it simply contains a bunch of maltodextrin. A rose by any other name...
In my mind, healthy equates with nutrient-dense. Maltodextrin, sugar, and fat substitutes just do not fit the bill. So if you're going to have dessert, have some damn dessert! If you're going to eat ice cream, eat real ice cream. Make it worth your while.
I know a few companies, Hagen-Dazs in particular, have created ice cream based on only 5 ingredients – cream, milk, eggs, sugar, and a select flavor; that's it. I don't know about you, but I appreciate simple foods, not to mention actually knowing what the substances are that I am eating. Obviously, real ice cream isn't low in calories, but it's supposed to be an occasional treat, and you really shouldn't be pounding down too much of it anyway.
While I can't point to a litany of randomly controlled trials showing that these foods are problematic, I can point out correlational research indicating that they are. When people replace regular soda with diet it doesn't lead to weight loss, despite the decrease in sugar and calories. Diet soda has also been linked with an increase in pre-term delivery, increase in cancer, and more.
Can there be a time and a place for these foods? Of course. Should you consume them on a consistent basis? I wouldn't advise it. Maybe that's just me, but I simply prefer real food.