In my experience NRSAs do not always fly "for free." I am not familiar with DL's policies, but half of my family either works for or is retired from AA. The only free NRSA travel on AA (excluding company business) is domestic Y travel for employees and their dependents. All premium class travel costs money, all international travel costs money and all "buddy pass" travel costs money. It is a lot cheaper than buying a confirmed ticket -- a buddy pass ticket in international F is usually comparable to the cheapest restricted Y ticket publicly offered on the same route -- but it is not free.

Also, unlike revenue passengers, non-revs usually have to follow a dress code -- generally smart casual in the back and business attire in the front. I did several very long flights in Y back in my youth, wearing a suit and tie on the off chance that Y would be full and I would be forced into C or F.