my knowledge is basic but i will take a stab at this. if you have stock tires and are riding in "light"terrain areas then you probably dont need to waste your money on a clutch kit.
i believe the main thing a clutch kit provides is a stiffer clutch spring. if you are running oversized tires, or slippery areas, (ie mud and sand) or really rough terrain (large rocks) then a clutch kit would probably help reduce belt slip, reduce overheating, and possibly a noticable gain in low-end torque. it makes everything just a little more "beefy" when it comes to the clutches, but is possibly a waste of money depending on riding location, style, and tire size