When my husband was growing up, his family lived on a farm in a very rural area, and all their doggies had free roam. However, very few of their dogs lived very long, because they either ran off, got hit by a car, or caught something from a wild animal. My husband estimates that about thirty dogs came and went during the 18 years he lived at home!
The cars are the main reason I could never consider letting my dogs off leash - we are in a city now; it's too dangerous!