I agree that this can be done. However, I think most people tend to underestimate the calories they consume. In fact, studies have shown that people think they are eating way less than they really do. What they might consider as a portion, might, in reality, be big enough to be 2 portions and double the estimated calories.
If a person keeps a food log, even for a week or so, and gets an idea of how many calories they are actually consuming, I think it's easier to see where to cut or where their problem areas are.
Most people can't just eat veggies and survive in the long term but if they've counted their calories, they'd know if they have enough room for a desert, for instance, that day.
