The resistor plug is intended to minimize EMI (Electromagnetic Interference). The more sophisticated the bike's control module, the more you have to be concerned about EMI. If I owned a something like and '01 YZF250, I'd be careful about sticking with a resistor plug, but IMHO, the KDX is not that complicated. I have put on many miles using B8ES plugs without any ill effect. I've seen one or two posts by KDX owner's reporting a high-rpm miss when using a non-resistor plug, but I have never experienced this myself. I currently run a BR7ES - since if you're buying a new plug and a resistor plug costs the same as a non-resistor - why not make the owner's manual happy? If, however, a case of new B7ES plugs fell from the sky, I would not hesitate to use them.