Ever since I took an AR15 class from an retired Marine I always of think of the chicken wing. That is what he kept telling me ... get that chicken wing down every time I shot the AR15. That being said, the picture from the movie a few shots above made me think of that. Finally a movie where someone isn't sticking their chicken wing out while shooting an AR15 / M4.
Why change techniques? Why not just use the best method?
Having your arm locked in tight against your torso takes more muscle error out of the equation. In addition to the obvious - presenting a smaller target area. Additionally, it takes more time and (wobbly) energy to swing that elbow up to wing position than keeping your mass in tighter to your COG. Erik Buell probably knows a thing or two about that.
While we're at it - in that pic the shooter has his pistol grip way back jammed up against the mag well. The further forward you have your pistol grip, the more control you can have. This is contrary to the COG argument but true. If you're holding a sword with both hands on the handle, and I grab the sharp end, I can move it all around. But if you have one hand on each end of the sword, I wouldn't be able to move it as much.
I wouldn't want my arm at full lock like that. More like the position of your non-dominant hand when you're shooting a pistol. But try shooting while moving, first with the grip in closer, then with it a little further out (shoot enough to give it an honest try). You shouldn't be carrying the gun with that hand. at any point you should be able to drop that hand because the weight of the gun is pinched between your dominant hand and shoulder. It's definitely not more tiring.