Plain fat free Greek yogurt is amazing, lots of protein, I use it to substitute something I would want to add butter or cheese to, it tastes like sour cream. I love it. Plus you can use it to make salad dressing and sauces and all kindsa stuff.
I think people here has so many different believes and diet styles that you will hardly find a conclusion. But from my experience, Yes yogurt is a strong source of calories, plus the milk contains galactose that also turns into additional sugar that will be running on your bloodstream and end up in reserves. Also fat.