If you have no problems with the stock bike running stock components, I can't see why anybody would want to buy a PCII. I agree with you, Eurodat. If it aint broke, don't fix it. If you're going to change anything major in the engine department though, air-box, exhaust etc, you will need a Yosh box or a PCII to remap the ECU.
Like DLS said though, RPMpower, the Yosh box is the tool you use to modify the stock ECU map. The PCII sits over the ECU control system and modifies the signals sent to the fuel injection system after the stock box has made the initial computations.
In my opinion, though this is contoversial, the PCII is better because it allows the fine tuning of the map in very small increments, the Yosh box is much more crude.