I've compared spec sheets for both QS and double-shield and the additional shielding means squat as far as attenuation is concerned.
On top of that, the 950-2250 MHz band that DSS systems use between the LNB and the reciever isn't subject to nearly the levels of ingress that cable TV, which operates between 5MHz on up to 860MHz or more, is.
Paging transmitters always do a number on channel 18/19 if there's a problem with the cable. Not to mention what ingress from CB transmitters can do to the return band..and then there's HAM transmitters, fire/police/ambulance transmitters, and let's not forget the FCC's cumulative leakage index requirements....
Both Cox and Comcast use quad-shield cable. Adelphia was last I saw still using double-shield, but that's probably all they could afford...
I should mention that Dish installers WILL use quad-shield--when it's already in the walls, installed there by the cable company. (My house was pre-wired (when it was built in late 1994) with quad shield by what was then Jones Intercable).