They are vegetables, it is true, but they have a big disadvantage: they can be eaten only cooked. The cooking process is ruining up their advantage.
They also have a high amount of calories in comparation with other vegetables. While the average of vegetables is 40 calories every 100 grams, they have 90.
Their calories amount is not the one to be worried about, but the situation makes them to be a bad idea.
Let's take the example of fried fries. Not the potatoes are the worst, but the oil. Let's take 100 grams of potatoes. When they are heating, they will get dry and the water will get out, that means that we will have only 50 grams of potatoes with 90 calories. Well, that is start to sound bad. That means we already have 180 calories for every 100 grams.
Let's go further, now our 50 grams of potatoes start to be filled with oil. How much oil can the dry potatoes can take ? On average, they will drink about 30 grams of oil. This means we now have 80 grams of potatoes at a rate of 360 calories. What this means if we eat 100 grams of french fries ? A meal of 450 calories. How bad that sounds ? Can you eat only 100 grams of french fries ? You have to eat a sandwich too which means you will have a fat meal.
A better way is to find other way to cook the potatoes, but don't eat too often. A meal with potatoes is considered a high calorie meal either way you cook it.