I agree with a lot of what Keri78 said, so I won't repeat all her discussion on quality of life for them, except to say ditto!
We only have 4 hens and we would be hit hard if something happened to one of them, but we see them free-ranging and foraging while we are home and they do so well. They aren't as "stupid" as some ppl think. They react to noise and threats and act accordingly. I will leave them inside my fenced back yard for sometime while gone during the day- I would probably not leave them for 8 hours everyday. I only let them out of our inner fenced yard to roam about the country side when I am home, this is because of the threat of stray dogs.
The strays out where I live are definitely scared of being shot- you can tell the way they act theat they've been shot at- so I scare the crap out of every stray I see and don't see them again!
I think the OP should assess the risk in your area and decide for yourself about them roaming. Do you have shrubs, low grasses, trees for them to hide under?
I think you might discover the the worst threat of them free-roaming is the chicken poop!!