Well, I don't agree. They contain little nutritional value and can easily be substituted by other and better stuff. Lentils (red, green, black), beans, pearl barley, eqq plant, cucurbita, cauliflower, broccoli and so on. But of course there's nothing wrong with having potatoes once in a while.
And, well, eating too many <insert any edible thing> will cause you to gain weight.