Yesterday, I photographed Danish actress Christine Sander. I'd brought an external light, but I decided against using it. Instead, I relied on the sparse window light, turned the ISO up to 2000 on the D800, and shot the 50mm f/1.4 either wide open or close to.
Like any portrait photographer, I obviously like shooting with external lights every now and then. But for a shoot like yesterday's, I'm even more thrilled to just use available light. Especially with the D800's ISO performance and the combination of VSCO Film and Lightroom 5 for the editing afterwards.
At least in this case, I'm not sure I'd have gotten better results with external lights - except in the ISO department, of course.