S&H helps increase your strength. After a 2-3 week rotation, I can usually lift more in my other strength workouts. I have read, however, that slow lifting really isn't any more beneficial to you. It FEELS like it's doing more, but isn't really. It just takes more time to complete the workout. I've read that from a couple sources.
If you do decide to use them, I would do a 3 week rotation every few months. When you feel like you're hitting a plateau.