Article from an NGK warranty claims officer, particularly applicable to RM's & KX's.

"Thank-you for your recent inquiry about the Suzuki RM125 and the recommended spark plugs for this application.

First I would like to explain why the R6918B-8 is so expensive. This particular spark plug was designed and engineered for this specific application. Due to the fact the production volume is lower and the number of spark plugs sold is much lower then that of the BR9ES, the cost of the spark plug is increased. The production costs and the
fact this spark plug utilizes gold palladium, a precious metal the cost is much more.

The Suzuki RM125 is a high revving, high compression engine and has vibration issues throughout the power band. We've tested with Suzuki and found the ES, EG, EV and EGV and determined these spark plugs fail under certain loads and conditions. If you compare the two spark plugs you'll notice a major difference in ground electrode design.
The R6918B has an electrode designed to withstand severe vibration and possible detonation associated with these bikes. A standard spark plugs ground electrode is welded on the outside of the metal shell and can vibrate like a tuning fork throughout certain rpm ranges. This severe vibration causes fatigue and the ground electrode fails. This is why the R6918B was chosen for this application. The R6918B is very reliable and durable in these conditions. Ignitability with this spark plug is better then the ES series due to the diameter of the center electrode. Any time you have a racing engine you need to choose a spark plug designed to withstand this type of environment.

I would also like to inform you I am the person who takes care of damage claims at NGK Spark Plugs.

I have had more BR8ES and BR9ES spark plugs
cross my desk every month with broken ground electrodes and receipts for repairs. When the ground electrode fails it bounces around leaving indentations in the cylinder head and the top of the piston. If your lucky the ground electrode will sneak out of the cylinder, however if your not the ground electrode wedges itself between the piston and cylinder wall and intern destroys the engine.

I hope I have helped you in better understanding why this spark plug is used in this application.