The biggest advantage to having pure N2 in your tires instead of 'air' is that if you have steel wheels, there can be no internal rusting. And I _have_ had wheels rust enough to cause the bead not to seat properly. Which required some poor kid to spend 2 seconds on them with a grinding wheel -- I don't think I paid extra for that, it was good experience for him anyway :)
With that said I would never pay extra for it, but if the tire shop offers for free, it's better to say yes. It's not worth more than $0.00 for most of us, but if you've got a numbers-stock '57 Corvette and you want to keep those original wheels for the next 50 years, might be worth a couple bucks to put an inert gas in instead of 20% oxygen, but I can't think of any other case.