I'm surprised that no one has called my own favorite uses for 'S'. I will use it for short bursts when I know or suspect the possibility that I'm going to need Warp 3+ pretty shortly and toot sweet, for example:
1. Merging onto the freeway. You know that jerk at your eight o'clock who won't speed up, won't slow down, and is dying to see you have to stop on the onramp? Feh. A short burst of full pedal in S and he's a little tiny speck in my rearview mirror. If I'm on an onramp and traffic is flowing freely, I'm in S. Once I've merged successfully I immediately drop back into D.
2. Approaching a red light and there's a slow-looking beater pickup sporting a confederate flag in my lane, so I switch to the other, empty lane. Then I notice that just the other side of the intersection the road narrows to one lane. So drop it into S and make beater pickup go away.
3. Turning right or left into speedy traffic. S is definitely your friend here.
I don't do any mountain driving so I don't have many other occasions to use S. The above applications don't require being in S for more than about 5-10 seconds, so the effect on total fuel economy is pretty much nil.