Copper plugs have a thick core which allows for a fatter spark line. During increased boost or cylinder pressures the spark will not "blow" out like a candle.
The rason platinum and Iridium plugs are not as good as NGK copper plugs for "high performance" applications is the spark line is thin and while it can be strong it is prone to "blowing" out at high pressures.
Also platinum and iridium are known to add to higher cylinder temperatures as they burn hotter. Iridium and platinums are fine for your application where you are not making any serious power.
The only reason platinum have been in the mainstream is for a mere convenience factor as the replacement interval is much longer than the typical 30,000 mile life of a copper plug.
Iridiums have been proven that the only place where they are an improvement over a conventional plug is at idle where it can save 1.5% fuel economy. I dont know about you but when i get in my car i dont spend most of the time at idle.
So the offset in price is not worth the extra benefit of better idle fuel economy. You are a victim of strong marketing. Ask yourself this how many times can you reinvent the wheel? Same concept, there hasnt been a major innovation in plug technology in a long time.