I think you will find all spark plugs used in RC are resistor plugs. If the part number stamped on the plug has an R in it, it is a resistor plug. Yes, it is almost a necessity for interference free ignition, even when using the plug cap and shielded wire. I have not used the plug cap on any of my planes (at least 10 different ones over the past 12 years) and using regular PPM 72 MHz systems and have never had any ignition interference. These were, with one exception, all magneto ignitions.