When the tires are mounted they have normal air inside at 1ATM and 78% nitrogen.
If my physics intuition serves me when bringing it up to 3ATM (32 PSI above ambient pressure) using pure nitrogen you now have 278/300 nitrogen or 92% nitrogen. (if there are any physics teachers or majors out there, please correct me).
You're going from 22% "other gases" to 8%. I suppose there might be a difference in performance but I suspect none that you could perceive on a passenger tire.
As far as being more friendly to a tire, I never saw dry rot on the inside of a tire. Sunlight, ozone, and oxidants outside will do a lot more damage that additional nitrogen on the inside can't affect.
I call "Snake-oil." Placebo. Additional dealer profit. But if it makes you feel good, by all means go for it.
Edit:
Couple links that pretty much echo what I said. They do a "purge" process where the tire is filled and "emptied" several times to achieve a 95% nitrogen mix, but they conclude that you're not going to tell a difference on passenger tires.
Consumer reports article
Consumer reports faq