Oil-based salad dressing usually contain soybean oil olive oil or canola oil- all desirable oils that are made up of mono and polyunsaturated fats- the good fats.  Having low fat dressing deprives of these nutritious oils.  Try avoiding creamy dressings.  Salad veggies are filled with terrific nutrients like lycopene and beta-carotene and your body requires fat to absorb these nutrients.  A small amount of nuts, seeds, olives or avocado would also add healthy fats to a salad.