Platinum plugs bad??? no, but?
The real issue is this: under very light load conditions (like idle, cold slow driving, etc.) the platinum plugs can foul because the electrode just does not get hot enough to keep from forming carbon deposits. During combustion, there are lots of carbon 'free radicals' around hunting for cold surfaces on which to collect. (lots means enough to leave a film that conducts electricity) Does no harm on heads and pistons because they warm up and burn off. BUT on the spark plug electrode, it CAN cause a partial or complete misfire, causing a rough idle and high emissions.
It depends on how you drive. A periodic 'Italian tune up' at high enough loads to get things hot in there helps keep the plugs clean and free from fouling.
Slow trips, frequent cold starts, no higher speed running to burn soot off plugs will cause them to foul.