Are you talking about mating a dry top to a skirt, or a single garment that covers your torso and also the deck of the boat (I think they're called dry-decks).
Playboaters seem to like them. If you swim from time to time you'll probably be more comfortable in a drytop and skirt, so that you don't get a ton of water flushing through the top when you swim.
If you roll every time all day long, and if the deck really fits your boat perfectly, then they're supposedly nice because they keep the cockpit drier.
The Kayak Wing