Never used S in 9 years of DSG driving, not sure what the point of it is.
I use D when stuck in traffic but most of the time I use the paddles in manual, that's why I added it and will always in the future.
The D and S modes are just a compromise, they are never going to predict the road ahead ie hills, junctions, overtaking etc. and it would be nearly impossible to do, I don't think a third mode would really help any.