My understanding is that a full twin-tip ski is simply designed to allow the skier to ski switch - nothing more. The compromise is that a twin-tip sacrifices running length, so a twin-tip skis like a much shorter ski that it is. No problem if you're just doing park and pipe, or taking off and landing switch on natural features outside the park. In addition, most twin-tip skis are lighter, softer in flex and don't have the torsional rigidity to hold an edge as well - desirable for a park ski, but not an all-mountain ski.
So, as an all-mountain ski for conditions where running length on the snow matters, the twip-tip is not as suitable as a ski with a flatter tail.
I have a pair of 190cm wizard Volkl Explosivs that have a partial twin-tip - that is, the tail is only slightly turned up and there is not much loss of running surface, so this design is a good compromise. I have had to back up on several occasions when I have been faced with unskiable terrain ahead, and this feature made it much easier.
I like to have my bindings mounted so that the ball of my foot is at the mid-point of the running surface of the ski. This position makes pivoting easier and quicker, gives me more agility in bump skiing and better feel in carving - because then there is roughly the same amount of lumber ahead of or behind the ball of my foot.
The compromise is that for high speed skiing, the optimum position would be about 1cm further back. However, its been a few years since I last did 100km/hr, so.....