The knocking thing is more of an issue for high powered cars (none of which you will find platinum plugs on though
). The common stumbling/stuttering issue is simply because they are piss poor conductors, and result in a weak spark. On a typical NT car, you dont need a ton of spark and the gains in life expectancy outweigh it anyway. On a turbo car, the mixture is much denser, and therfore harder to get a spark to jump through. The better conducting copper plugs privide this power, at the expense of a shorter life. Change your copper plugs every 10k miles at the most.