No, the focus should be eating the right amount of healthy food.
You shouldn't just "eat less" you should "eat better", and that means eating an appropriate amount of healthy good.
Eating better is obviously preferable, but the reason most people fail their diet is because it is too hard for them to comply with it. This is where the "switch to diet soda" is so effective; people can actually do it.
In humans, we have a lot of evidence that shows that the source of calories does not matter, for fat loss. Long term health is another issue, of course.
IMHO, putting focus anywhere but on calories is doing oneself a very big disservice, except for some athletes and bodybuilders for whom other factors are important as well.
The acid in Diet Coke is real bad for your teeth, but fruit juice is more acidic and has more sugar than diet soda and yet you probably think it's healthy, right?