...hanging off should only be necessary for on track speeds. If you're running that hard on the street, you're asking for trouble...
I like that, and agree with one proviso. Learning how to be fast is equivalent to learning how to be smooth, and how to be coordinated on the bike. And that's a desirable goal for all riders, and can be practiced at medium / safe / street speeds. If you don't know how to ride in the twisties with your weight centered on the bike and your body mass only on the balls of your feet, your knees on the side of the tank, and nothing but a light touch on the grips, your skill set has room for improvement. And improving skills can increase safety.
The best post in this thread is #110 on page 11 by Tunnelvision.
http://www.fz09.org/forum/6-fz-09-general-discussion/7060-how-do-you-lean-your-fz09-properly-11.html#post131102
I'm going to try to expand on what he said so concisely, but probably won't do as well.
First of two basic skills: After using countersteer to set up the turn, all riders should know to minimize their weight on the bars and ride fast with a very light touch. What a good rider should be shooting for (as early in the turn as possible, as he/she sets up and holds a stable line and lean angle) is to loosen his/her grip enough that the tire can choose it's own angle. On it's own, it will likely point slightly more toward the outside of the turn than a heavy hand would hold it. But learning to balance and control the rest of your body enough that you don't need your hands to balance or hang on lets the bars settle into the angle they want to be - it's a magic and smooth place that's faster and offers a higher level of grip than you could find with a heavier hand. All racers ride like this, but only a minority of street riders do. The real bonus is if you do hit oil, gravel, or sand and are relaxed enough to let the bars do what they want once that front tire slides the chance of recovery doubles or triples (if the tire is allowed to recover on it's own). All the weird physics forces out there incline toward stability. Your job is to stay out of the way and let the round rubber do what it wants to: keep rolling. Ideally, you'll get to a place where this is your regular way of riding. Keith Code has written about this extensively.
The second involves hanging off the bike. As Tunnelvision stated, a really important part of riding well in a turn is front / rear balance. A simple analogy is jumping a bicycle. You can land on the front first, or the back first, but when you land on both tires at the same time, you get the benefit of splitting the landing forces between the two tires equally. Leaning around a turn on a motorcycle is like that: there's the friction needed to keep turning (which is accelerating even without throttle), and you need to split that friction equally. The problem is that most bikes which are also under throttle acceleration in a turn shift the balance too far toward the rear. So getting up and off the seat is properly used as a way to shift your own weight
forward to put more traction on the front rubber. If you can manage to balance the rest of your body well enough that you aren't using your hands on the bars to balance, and you experiment on how far forward you're getting your butt forward around the tank, you'll find a sweet spot where you're splitting the suspension load between the sweet spots in the front and rear springs. The bike will feel suddenly right, and more capable than it ever has before. Generally, you can't get there sitting centered on the tank, and for sure not in an uphill turn. It's a really great sensation, best practiced on a track but you can work on it on a smooth and familiar curvy road. If you've never experienced the sensation, your skill set has room for improvement. Working on it will make you a better, safer rider.