Another reason is that in some engines, the spark has to also travel in the other direction and so the use of ‘double platinum’ (a bit on each electrode) helps with that.
That is debatable
I can show you any number of vehicles in our fleet, where that was decidedly NOT the case
I would use the specified plug and change it more often if I was concerned about the long change interval.
I agree . . .
Use the specified plugs, and change more often, if it makes you more comfortable
As for the plugs not lasting well for 100,000 miles, let me clarify
There seems to be a misconception out there that all platinum and/or iridium plugs are supposed to last that long
Not so
If you carefully read factory maintenance schedules, you’ll discover there are plenty of modern vehicles out there that use those precious metal spark plugs, yet the specified replacement interval is far shorter, 60K in many cases. And there are several vehicles in our fleet, where they started bucking like a bronco at just past 60K on the original plugs. Exceed the replacement interval at your own peril. That last statement wasn’t meant for you specifically 
My 06 Mustang 4.0 V6 got its first plug change at 101K with Ford specified plugs.
Ran excellent before the change, runs excellent after.
You’re absolutely right of course. I have a fair amount of expertise in electrode design and the application the plug is subjected to has a tremendous impact on the longevity. For example, given two exact same electrode designs subjected to different cylinder pressures and temperatures will have a dramatic effect on the wear rate of the electrode. Also, the peak power applied to the electrode has a significant effect as well. So a higher energy ignition system will wear plugs faster than one that uses a lower energy level. To dig a bit more, the peak voltage matters less to erosion rate than the peak current even if the power (area under the curve) is the same. Like most things in life, it is a system and needs to be analyzed as such. A plug is not a plug when it’s placed into two different system applications…
Contrary to what many believe, the manufacturer has invested in analyzing the impacts to longevity in their application and come up with a recommended change interval. Sometimes, that ends up being very conservative because only failure is definitive. On the systems where it is 60k and they fail in around that period, the analysis and subsequent test verification exercises were probably very definitive. For 100k, they may not have seen many, if any, actual failures but the total elapsed time in use and potential for issues during removal then dominated the recommended service schedule…
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