With respect, Mechaniker, the only reason for putting nitrogen in street tires is as a sales gimmick. Some stores have also charged for it, which I find appalling.
Air is roughly 77% nitrogen to begin with. And, when "filling" the tire, shops don't purge the air, they just add the nitrogen to bring it up to pressure. Which means the oxygen, argon, and misc that comprise the other 33% of the filling of the tire is still in there.
In addition, I've never seen anything analytical that proves or even suggests that tires with air in them deteriorate at a faster rate than tires with nitrogen in them. Perhaps in a lab it could be demonstrated that
I read the scientific reason, but it does not take into account that we're not comparing nitrogen with oxygen.....we're comparing nitrogen with 77% nitrogen, 31% oxygen, and 2% argon etc. And the nitrogen we're "comparing" is only added to the air in the carcass, not replacing it. And, even the article acknoledges that no information exists on how much difference it makes.
Racing tires I cannot comment on, except to say that they operate at far higher temperatures and are far more sensitive to pressure changes, especially running at their edge of adhesion. And that nitrogen will not feed a fire in the pits as oxygen would.
In raod tires it's a sales gimmick, pure and simple.