I run my A/C almost all winter, but then I live in Florida. Even then it does get cold once in a while.
I normally do run heat and A/C at the same time when its cold. It keeps the inside air dehumidified, and the windshield clear. If it's warm in the car and cold outside, the inside of the windshield will fog up from just breathing if it's not set on Def. And yup, the A/C comes on when you set the switch to Defog/Defrost.
The night Irma moved in with us, I needed a Defrog setting. Some of my neighbors still need a Degator setting.
130 total pounds of Pitbull ruled by 8 pounds of cat.