It depends. With some of my skirts and dresses, the slip just works up and makes kind of a mess. On the other hand, with some skirts, the slip is nice because it lets things slide around and helps stuff stay in place. In the winter this is especially true. If you have a thicker, warmer pair of tights, there’s a real danger that a skirt will cling to your legs and ride up. You’ll be adjusting it constantly. With a slip, everything slides around and sits where it should. I have a leather “paper bag” style skirt with a thin fleece lining. It’s super warm and comfy in the winter, but over warm leggings it clings and rides up. With a slip, it sits perfectly.
So, short answer, I wear a slip when the wardrobe requires it.