Smooth skin is used at the outer layer of "high end" spearfishing wetsuits, where they don't mind buying a new one every year or two. They have no liner or fabric, open cell inside and smooth skin outside. What in spanish is called "Chicle" and "Liscio/spaccato" in Italian.
Some manufacturers have "Sandwich" type neoprene. This is open cell inside, Lycra inner layer and smooth skin outside.
Smooth skin is mostly used in cold weather, where you can freeze when moving from one spot to the other in a boat if you wear a lined wetsuit. Is the only advantage I see in thin thicknesses. When you talk about 7 mm, a smooth skin wetsuit is much more flexible and comfortable than a lined one.
