Well, it takes more energy to burn a fat molecule than it does to burn a sugar or more complex carbohydrate molecule. Thus, limiting fat intake without increasing your exercise at a fat-burning level isn't that good.
All the talk about fats/carbs/sugars has it's good and bad points, but burning more calories than you intake is the key to losing weight.
Having a decently well-rounded diet is also relatively important in terms of genereal health. Too much of anything is not really a good thing, be it carbs, fats, sugars, whatever.