Have to agree with Scrap. I used to stand in one place and smother my punches on the heavy bag, constantly reseting every time the bag didn't sit where I wanted. Now, I move with the bag as it sways. I now manuever and pivot around the bag practicing footwork and developing good range for puches rather than smothering them.
Focus mitts are crucial because they develop your combos and the pad holder throws back at you as you develop more defense.