Hi Steve,
 
If you do a quick google search on plug materials you'll get your answer. Copper spark plugs will always produce a much stronger spark which is why they're advised for high performance applications. Platinum and Iridium plugs provide a spark thats not as strong but they're far more accurate when they spark and will easily outlast any copper plug due to the metal content. For these reasons manufacturers will use a platinum or iridium plug as they don't need the strong spark with factory tuning and have that longevity as you mentioned. Anyone with a FWD V6 will usually find that the front bank will have different plugs to the rear bank because its a more difficult job and rear bank can last longer with the platinums i.e. Magna's. 

I can't speak for Toyota when they were picking the plug but I dare say at least 70% of their attention (being Toyota) was on longevity and the platinum is a result. 
 
Feel free to shoot me a message if you want to talk about it further,

Cheers :)

Have some pages that expand on this that I quickly googled:
 - http://mechanics.stackexchange.com/questions/2013/what-is-the-difference-between-normal-platinum-iridium-plugs
 - http://www.carsdirect.com/car-repair/copper-spark-plugs-vs-iridium-and-platinum-understand-for-top-performance
 - http://www.bobistheoilguy...lat&Number=2583605