For me rearsets are absolutely a necessity. I have a set on my ZX-10R from Attack. On the ZX-10, the levers are integrated into the rear set. With aftermarket rearsets like my Attack, the use of better bearings at the pivot point for the levers makes shifting much smoother with less slop. On the Ducs, this doesn't factor in, but I thought I would mention one of the benefits which applies to other bikes.
The main reason why I want aftermarket adjustable rearsets is proper leg positioning. With the stock rearsets on my 848, the stock rearsets places my knees at the wrong spot. I find myself pinching in at the point below the fuel tank where the black side panels meet the tank. This prevents me from getting a positive lock on the tank for both braking and cornering.