A sweatshirt might be considered as an unstylish garment by some. Yet I think that if it's a plain sweatshirt or with a cool design, it can help create a good ensemble. One should always wear a sweatshirt if the occasion and weather allow it.
Below a photo taken by Richard Avedon of Rene Russo and Tony Spinelli, where we can see them wearing a sweatshirt and a tunic sports jacket. Woth noticing is the impressive background!