If the rate of leakage of oxygen is significantly greater than nitrogen then after a few fills most people will end up at 95+% nitrogen anyway provided they don't let their tires drop more than a few PSI, and if they're letting their pressure drop more than a few PSI anyway they shouldn't be worrying about what they're putting in their tires.
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Note also that if you fill and top off with air, the percentage of N2 will gradually increase over time; at first you lose about half O2 and half N2, but you top off with a mixture of 78% N2 and 22% O2. So if you fill your tires with air and they initially lose about a psi a month, then after a year of this, it’s down to mostly N2 inside the tire, and the leakdown rate should slow down. For many motorcyclists, tires don’t make it through a season before replacement, so the effect is negligible; but if your car tires last four years, then by using air in them you’ve actually gotten about 3/4 of the benefit of a N2 fill, without the expense.
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Link.