Turn the question around: given a lens which costs $1500, would you pay another $500 for the option to have it repaired in the future by a U.S. repair facility?
You can get a non Nikon USA imported lens repaired in the US, you just can't get Nikon USA to pay for it under warranty nor have it done in a Nikon USA Authorized service center. You can even get it fixed at the same shop, by the same technician who does 'authorized' warranty service as long as the shop keeps a paper firewall between the 2 sides of the business. Since I buy from B&H most times, that is where I would send a 'grey' lens to have it repaired. Currently, I don't own any. Last 2 lens purchases were Nikon Refurbished lenses with only a 90 day warranty. I'm 3 years in now.
In 30 some years, I have had 1 lens that needed repair because I dropped it onto rock. I'm not hard on my stuff, but I actually use it regularly. My D300 had 157,000 shutter actuations when I replaced it with the D7100 which has over 85,000. Haven't checked the D810. I never use 'protection' filters except in extreme conditions because I just don't go around poking stuff at my lenses! They have hoods on them when appropriate and lens caps when appropriate.
Nikon USA lenses have a 1 year warranty which they will extend to 5 years if you register the lens. All those lenses on Ebay that say they are warranted are not because to get the 5 years you have to register it and as soon as you do, the warranty becomes non transferable!
My Bottom Line: If I am buying a lens for the warranty, I'm probably buying the wrong lens. A lens very rarely goes bad unless you abuse it. I have an insurance rider on my equipment for theft or loss. While IS and VR and focusing motors add to the likelihood of a lens needing repair it is rather rare.