As Phil indicated, its becoming more neccesary in New York if you are shooting from the perimeter, business or no business.
Coincidentally, I ran into Dariusz Jewezski (PhotoDj) today and got his card. He was on assignment, and would have a card anyway, but at this point it would be helpful for anyone pointing a lens at aircraft.
Its funny to think...nowadays spotters exchange their business cards... a few years ago, not so common I don't think. Perhaps the serious slide traders did; I don't know.
Not sure if Art Brett still uses the same card for his business, but last year it featured a smoking AeroMexico DC-8!