I agree the rubber end is there primarily to block water ingress. But, it also blocks dust, dirt, and mud from coating the spark plug insulator. Any coating of a conductive nature will form a path to shunt away spark energy.
If this is an issue, you should be able to see it arcing in a darkened garage.
Spark issues will often show on the plug internal tips as different deposit patterns, especially on the effected plug.
Anyway, you know it should be there. Why are you avoiding having the proper equipment?
Electrically, the spark circuit only cares about the total resistance in the circuit loop, and doesn't care where it is distributed. The coil contributes about 15k ohms. Adding 10k to 20k, with plugs and caps is rather normal and improves spark.
The spark plugs fire in pairs, so the resistance in two leads and two spark plugs are considered in the calculation. Resistor spark plugs usually have a 5k resistor to consider. Most run these with 0 ohm resistor caps. But, a healthy spark loop can still be acheived with a 5k cap also installed. Not recomended to use 10k caps along with 5k res plugs for the stock ignition circuit, as that would add 30k ohms to the spark loop circuit, which would limit the spark current considerably.
Cheers,
