ordinarily, i would let duke's comments ride, but he has it so wrong that i am compelled to interject some strands of engineering verities.
i haven't checked out the plugs or high voltage leads from my newer benzes[e320cab, s500c], but my 116[6.9] and my 126's[560sel, 560sec] do have resistance in the secondary circuits. the resistor[4k-7k Ohms] is potted into the spark plug boot.
furthermore, though i haven't examined my distributor rotors lately, years ago, there was a resistor potted into that component.
i dealt with this some weeks ago in a post asking about 1k or 5k ohm resistance.
succinctly, d-b understood that arcing areas needed rfi/emi suppression or attenuation. and by and large, these levels of impedance are negligible in the overall scheme of the inductive ignition systems that were being used.
as to multiple electrode spark plugs, that they are not used as original equipment is basically a matter of cost. automobile manufacturers are going to select the least expensive component. more than a single ground elecrode costs more money. qed.
in the realm of the most difficult spark-ignited engines, severe service, methane-fueled engines that run continuously at elevated bmep's and are most often turbocharged, multiple electrode spark plugs are the most commonly used. they have no performance drawbacks in comparison to a single ground electrode spark plug. they furnish considerably advanced levels of durability.
as an example, a single ground electrode, precious metal-attached plug has been fitted as original equipment for cng-fueled diesel conversions. it will make emissions numbers, but durability is generally limited to less than 20,000 miles. and unpredictably so. a multiple ground electrode plug in the same engine will make the same emissions numbers, but will live predictably for more than 100,00 0 miles.
quite candidly, engine manufacturers do not go to great lengths to evaluate alternatives to the cheapest or most graft-provoking components.