why exactly is it bad for it to have high caloric density?
Well the danger comes in when the person is inexperienced with calories/dieting. This makes foods with high carb caloric density easy to overconsume whether the goal is losses or gains.
If you are trying to gain weight, and you eat 3 servings of oatmeal a day, its very very very easy to stimulate an insulin response by overeating the high carb food and you will find substantial fat gain in addition to your mass gains (assuming you are getting ample protein).
If you are trying to lose weight, you are likely to overconsume on something that has a high caloric density and see stagnant weight loss.
For the sake of simple living, you get tons of vitamins and minerals from vegetables and it is nearly impossible to overeat.
If you are REALLY on top of your shit though -- measuring and counting every calorie -- then the caloric density of food doesn't matter -- except in that you may find that many of your meals are VERY small.