A big argument by the nitrogen-in-the-tire crowd is that:
Nitrogen atoms are bigger and thus less likely to escape the tire, bringing stability to your tire pressure.
If Earth's atmosphere is %78.084 percent nitrogen, then the non-Nitrogen composition is ~22%. If this 22% is more likely to seep out of the tire, and Nitrogen doesn't seep out, then simply filling the tire up with air will maintain the original 78% N, plus the added nitrogen which would be 78% of the remaining 22% (assuming total seepage).
Simply, won't a tire that's been filled up 10 times in the course of its normal life already be disproportionately Nitrogen if this argument is true?