they r excellent for wintertime if the weather is cold enough.
a cotton turtleneck underneath a sweater is a good warmth-upgrade to wearing a shirt under a sweater and gives a similar look. same with a wool/cashmere/etc turtleneck underneath a sportcoat. a scarf is great when u r outside facing the weather, but what about indoor spaces when there is a chill in the air (think any old building for a reception, party, restaurant, etc). and when its really cold a turtleneck and scarf is the only way to stay warm.
its mid dec, snowing lightly, dark at 330pm, u r cold and tired of shopping. you duck into a pub/bar/cafe/etc to sit near the fire, enjoy a dram or two and warm up. clearly u r wearing a turtleneck. and feeling better for it.