I agree with Tracie. It makes a big difference whether you are just pleasure sailing or racing. For racing, you really need a full harness, because you have to get OUT there, full stretch, and you can't do it without the back support of a full harness.

If you are just out there for fun and using the sails to balance the boat and want to do the famous "Hobie sit" (as Randy Smyth refers to it), a butt-bucket is fine.