If you have coyotes or other wildlife, they really have to work on it for a long time to even think about getting through. A pet door allows the pets to come back inside should something outside the fence scare them.
Cats were meant to be outdoors, running around and roaming and playing in the fresh air and sunshine. They're much happier. These kittens we have outside within the enclosure just tear around the yard, tackle each other and play.