Your husband must of confused chickens with feral pigs, chickens do not ruin yards. Ducks will drill tiny holes through wet mud with their beaks and when the mud drys you will have a tiny beak sized hole n the ground but it goes away pretty fast. I live in Wisconsin, if you let a flock of 25 birds free range all day every day and only shut them up at night, your flock would be dead within 2 years. If you do want then to free range then I would suggest letting them out an hour or two before dark, then just sit out there with them until they go in for the night. 
    The rooster will let you know if any predator attacks, they have two alarm calls, both are used for several different things. The first one is to make the hens look up into the sky, he makes this alarm call every time a animal flies overhead. If the animal is a hawk the hens and rooster will run for cover under bushes and shrubs, they will hide until the hawk leaves. If it is another bird like a dove or songbird the hens will stay alert afterwards but they will not run. Young roosters will even make this alarm call when a airplane flies over. This call sounds kinda like a crow but instead of CockaDoodleDoo it's just a really long Dooooo.
     The second call is an alarm call for when a rooster spots a predator on the ground, it alerts the hens and makes them watch the predator. If the hens and roo can keep track of where a predator is then it is much easier for them to avoid when it attacks. This alarm call is also used for when a hen is caught by a predator, all the chickens will again watch the predator, they will keep their distance and will calm down a while after the predator kills the hen and leaves. The rooster will get really close as he makes the alarm call, he will try to intimidate the predator, he will also alert everything in the area that there is a predator. Some roosters will even attack the predator but they will usually end up getting killed to, depending on what it is. This alarm call is very loud and sounds like a loud and deep cluck, similar to the cluck some hens make after laying an egg. The rooster will also make this same call if a hen starts to make it, if you let your birds free range and a hen leaves the flock to lay an egg when she is done she will make this call to notify the rooster, the rooster will call back with his louder call and it will notify her of where the flock has moved to in her absence. If you don't have a roo the top hen will make the cluck alarm call but most hens are incapable of making the the "Doo" alarm call.
   So yes, you can let your birds free range, no they will not destroy your yard. It is much more safe to have a large run for your hens, but if you do let them free range then make sure you stay out there with them, you do not have to follow them around, you can just sit out on your porch and listen for the alarm calls the rooster makes and then when he makes them just go and check to see why he has made the alarm.