Calorie for calorie, fruit packs more healthy nutrients than most foods including important vitamins, minerals, phytochemicals, etc. Everyone's diet goal should be to do the best they can to eat healthy foods (e.g. whole grains, fruits, vegetables, lean proteins) within the amount of calories that allows a healthy weight or body configuration. Dieting by definition means consuming fewer calories which lead to fewer nutrients consumed. When dieting it becomes more important to select foods like fruits (1-3 servings/day based on preference and total calories allowed) with higher nutrients per calorie. As the saying goes, "you need and get more bang for the buck".
When dieting you are allowed a certain number of calories in order to reach your goal, so choose the foods that make you feel active, satiated and help cover your overall nutrient needs. And take a multivitamin & mineral supplement to shore up any low or missing nutrients, which is especially common during dieting.