I recently attended a training session by David Guy Maynard on using speed lights for location portraiture. Fascinating topic! He brought along a couple of professional models that we photographed after his presentation.
The problem was that twenty or so of us were standing side-by-side using our camera-mounted lights.
How boring. So I decided to crank up my ISO, turn down my flash, and move to the side so I could use his modeling light to backlight the subject. Much better!
Needless to say, I prefer these shots. Backlighting certainly makes for more interesting images.