Mach 1,
Although platinum does conduct more poorly than copper, that's not the reason why it has a weaker spark. The only time you're conducting through platinum is at the very tip of the plug. The tip is only coated with platinum, BTW. Platinum acts as a catalyst to allow a spark to occur at a lower voltage than your typical steel tipped plugs. They fire much more reliably because of this, especially with the wimpy coils that they put in cars these days. Since the platinum is also more durable, it holds the gap much longer. Current fuel economy and emissions requirements are what is driving their use, not just there durability.
I suppose we're all familiar with the plumbing analogies related to voltage and amperage. Amps measure the volume of electricity moving through the wire, and volts measures the pressure of electricity moving through the wire. Think of the tip of the spark plug as a pressure relief valve. A platinum tipped spark plug will pop at a lower pressure than a steel tipped plug. It will also pop with more reliability.
It's probably bad to say "use copper core plugs". It should be more correct to say "use copper core, steel tipped plugs".
The fact that we all can agree on is that platinum plugs have a weaker spark which is not what we want in a high horsepower engine with a high performance coil.
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351W 89 Mustang GT Convertible