There is 78% N2 in normal air that you breathe... that gets filtered out when they sell compressed oxygen (welding supply, etc... ) They had to do something w/ the N2, right? Why not put it in tires?
There is no valid reason for using N2 unless it's to satisfy a warranty requirement, or if it's free. To properly add N2 to tires, you either need two valve stems, or you need a purge the tire after seating the beads on the rim, before adding the N2. If you don't, there is always going to be "other" in the tire/rim assembly's airspace.
N2 also is supposed to not leak as fast as breathable air from your tires, because the N molecule is larger than the pores of the vulcanized rubber carcass of the tires... if that's the case, eventually, you should end up with nearly pure N2 in your tire anyway, after filling them a few times.
If N2 is 78% of the air you breathe, then it's 78% of the air in your tires... If your tire pressure drops from 100PSI to 80PSI, that's 20%, but all the N2 remains in there, so you're losing 20% of "other", which is O2, CO2, CO, etc... but keeping the original 78% N2. You add that air back in, and now the new 20% that you added is 78% N2 as well, so you've increased your N2 as well, making the new number something like 80% or more.
Keep doing this, and you'll eventually end up with almost pure N2. Why pay for it, when you could just inflate your tires again and again?