SNS3Guppy:
The fact is that in an automobile, the underinflation is the primary cause of blow-outs, largely due to heat rise. The reason for the heat rise is sidewall flex.
Like everything else, improper maintenance of safety equipment is just asking for trouble. Everything I've read states to check the pressure (when cold) of a passenger automobile at least once a month and with a high quality gauge. (Sadly, done by far too few people.)
With proper inflation and at least minimum safe tread remaining, plain old air is as good as nitrogen. But, if the automobile tire is seriously underinflated, sidewall flexing (as you stated) will eventually ruin the day, pure nitrogen in the tire notwithstanding.