one of the salesmen in the camera shop I used to work for would always say "P for professional" his comment was that the working pro understands the metering and focusing systems of their camera and knows when they can trust them and when they need to dial in some compensation or simply go manual. Sure working in A or S modes might give you the best of both worlds, but often times P is simply a faster way of working. glance at the shot, scan your eye around the edges of the frame, glance down at the exposure info and press the shutter in less than a second and if you need to dial in compensation you can usually do it on the fly.