Call me crazy, but I think rack pulls are a great way to improve the deadlift, along with other exercises of course. First off, you must be sure of your deadlift form, and make sure it works for YOU. KK pulls one way, George Leeman pulls one way, I pull my way. Keep in mind that the way someone tells you to pull may not be the best way for you to do so.
I can only say what worked for me, (in addition to lat pulls, cable rows, pull ups, and straight legs) was incorporating heavy (3-5reps) rack pulls without a belt from just above knee height towards the end of my workout. My reasoning behind it was that it would get me used to heavier weight. When you think about it, your body has to be strong as hell to be able to withstand the force of 700+ pounds pulling it down.
Rack pulls can strengthen those ligaments and tendons, as well assisting in the strength of your "valsalva" maneuver. *I believe you can strengthen those muscles required to utilize air inhaled into the diaphragm to stabilize the spine, thus, allowing you to deadlift more. That is why I do them with no belt. I have also noticed my traps get huge from them, and my grip strength improves. Just make sure you are still doing regular deads, because just like deficits, if you spend too long away from regular deadlift, you may start to forget your original form. Or, your body will grow so much that when you return your form *will be different. Hey, hope that helps.