It isn't a new thing at all, and the term has been around for a while. I was an Infantryman from 05-08 and heard it back then. I have also more recently been a defense contractor. I don't think members of the military prefer any title, honestly. In the most broad sense, good terms are soldiers, sailors, airmen, marines. Defense Contractors constantly refer to the military as "warfighter" and have for a while. In short, nobody in the military is going to flinch one way or the other if you use either term. Just don't call marines anything but marines.