It all has to do with your area and level of predators. I've been free-ranging several years, and strangely enough, the only predator that has ever killed my chickens was one that tried to get into the coop.
For me the only con was I needed more fencing than I thought... Chickens are sneaky.
Less feed consumed.
Insect population lowered.
Higher nutrition in eggs.
Less work needed on the coop. (You don't need a run if your birds are free-ranged....