I read a write up awhile back that claims the DS has a very advanced auto climate control system. It's sensors are reading a lot of variables. There's a lot more to it than you think. Auto CC keeps your vehicle temp at a very steady and constant temperature which you set. Jut because the AC light is on does not mean actual AC is running all the time. If you turn off the AC, effectively turning off auto CC, then the system will just blow the air temp that you have it set on at a constant rate and constant temp. Soon you'll find yourself turning the heat down...just like the "old days". I mean, who wants to go back to 1990 right? LOL!
Anyway, with the Auto CC on you never worry about that.