It does not take less voltage to jump a .32" gap in a given fuel mixture if the electrodes are irridium rather than platinum. The gap size, the mixture, and the mixture density determine the voltage required. Compression is a factor in the mixture density, as is the density of the mix that comes in through the intake valve.
In plugs with 30,000 miles on them, it perhaps does take more voltage to fire a platinum plug than an irridium plug, only because irridium does not evaporate (erode) as readily with arcing. Irridium can therefore maintain a more consistant gap and a more consistant spark over a longer period, allowing them to be changed less often and, I'd argue, making the engine run clean and more efficient for longer durations. This does lead to more reliability and less pollution long term.
However, I've always believed in staying with the manufacturer's recommended plugs for stock engines. While a plug manufacturer may list both platinum and irridium, and both may be physically interchangable and have the same heat range, the proper one ensures that no anomolies due to some suttle difference.