I can recall two times commuting when my slicks momentarily slipped. Once in the mud and grass I was forced to ride through due to flooding, and another time in fresh snow. This is so infrequent, it is hardly worth even mentioning. If your commute involves mountain biking, or climbing or descending steep hills in mud, yes to knobbies, otherwise, no. For snow, I use winter tires which are lightly treaded with carbide studs.