I think the moral to the story is this: If you let guineas free-range, sooner or later you're going to lose a few. There are things you can do to cut down on losses, like penning/cooping them at night, but even then there is a lot to worry about: hawks, neighbors' dogs, cars...Luckily, I don't have neighborhood dogs running around, and the only day predators I have are small hawks, my biggest daytime worry is the road. My nocturnal predators are the biggest threat. But that's my situation. Everyone should evaluate their own situation to determine what's best.
But here's how I figure it. I free-range during the day, and there are threats out there for me to worry about, like car accidents, falling off a ladder, your standard 'evil-doer'.....there is no guarantee that I'm going to make it through the day (I don't live in fear LOL), and I coop myself at night, at home in my room to sleep. If I were out all night, my chances of coming to harm increase. I'm happy and have lived to the ripe old age of 55, so it's a system that works for me. I think it works for my guineas, too. They live with risks just like I do, but I don't want to deny them a full and happy guinea life, such as it is. The same goes for my chickens.
But "stuff" happens, you know? If you really think you would enjoy and benefit from guineas, go for it, and do the best you can for them.