Selecting the correct heat range for the spark plug is a balancing act
between the speed of erosion of the electrodes (wear) versus the
accumulation of deposits (fouling). The correct range selection is based on
a few factors:
1. Engine duty cycle. The percentage of time the engine spends warming up
and at idle speed verses the time spent warmed up and at high speed  .
2. How rich the fuel mixture is.
3. How much oil the engine consumes.
4. The engine operating coolant temperature.
5. Engine compression ratio and the octane of the gas it is burning.
The spark plug only needs to be hot enough to stay clean. Any hotter only
increases electrode wear
and can contribute to preignition (pinging). Sometimes factors will cancel
each other out.
Any one of the following conditions  can make a hotter plug the corrrect one
for the application.
Examples are:
1. More short trips opposed to more long trips.
2. Richer mixture in high performance setups as opposed to late model
"clean" engines.
3. High mileage engines that use some oil opposed to newer engines that use
almost none.
4. Earlier engines that use a 165` to 180` thermostat cool the sparkplugs
more opposed to newer engines
that use 195` thermostats.
5. Later model engines use a lower compression ratio for lower octane gas
and have lower combustion temperatures opposed to earlier high compression
engines.
The platinum tip plug was 'borrowed" from the aircraft engine industry where
even the coldest sparkplug would not last due to the constant high rpm and
load of an airplane engine. It can do both jobs. It allows a very hot plug
to be used in a car engine and still have very long electrode life. This
means that the plug can be hot enough for a car that will be used for very
short trips where the engine never reaches operating temperature or constant
highway speeds that would wear out a conventional plug in short order.
In short:
You can use as hot a platinum plug as you want as long as the engine doesn't
ping.
You can use as cold a conventional plug as you want as long as the electrode
doesn't collect deposits.