We get a few requests for a fully decked harry, but have always managed to talk the client out of doing it! Not only would it not sail well, but there would be no easy way to sail it and no easy access to outside.. Where it might work would be on a boat big enough to have the cockpit on the cabin roof, but this would be more super yacht than harryproa.
It would be easy to fill a harry with bunks. The reason we don't is a deliberate attempt to keep the relationship between the numbers of people who can comfortably live, sleep, have dinner and party on the boat. Bunks for 8 on a boat with only 6 seats at the table is pretty silly; a party where a few people have to stand squashed together in the cockpit is not as much fun as a large number being able to sit comfortably (ie, on seats rather than on the floor) around the deck, etc.