Sometimes you would want to position your subject off center... or some place in the frame that is NOT covered by any of the 39 AF pts.I understand from the manual. However if I fix it at 39 pts, then I don't need to adjust in any scenario. So what is the trade off if using 39 pts for portrait or still picture ? Slower focusing ?
Also it's usually the center AF that's the most accurate of the lot so some would prefer to use center AF and do a focus-recompose (i do this 90%+ of the time when I was still on Canon 5D Mk2).