I've used it some. It's useful for working with students who have poor control, or who are new to a specific tactic. Have to use it sparingly, though, because it can create bad habits on both sides. There's really no need to defend a shoulder much, so it doesn't really mimic defending the head. In fact, even if you do defend it well, it's not much like defending the head and usually causes you to leave the head more open in the middle. And, of course, you can hit the front shoulder from further away, and with different openings than the head.