I can see both sides of this discussion. I have two "barn cats" living in my house. I live out in the country and my cats go out the back door, away from the road. I know I take a chance because they go out but they sure want to go hunting. My oldest cat (5) started her life in a barn until she "found" my house. She's a great little hunter. I describe her as a house cat who happened to live in the barn. My other cat was a drop-off that I was unable to home through the vet's office.
Cute as they are, both cats have been spayed because, frankly, I don't want litter after litter after litter. The gray cat is delicate and repeated pregnancies would just suck the calcium right out of her bones. My financial resources are concentrated on these two--and another outdoor cat that came from a neighbor's--and I hope they remain safe and healthy, and I work to keep them so. My gray cat, Poppins (named for the practically-perfect-in --every-way Mary Poppins) really doesn't want any additions, and is particularly resentful of any cat that I love on, esp. the "barn cat." He's been neutered, because I didn't want him "traveling around."
I'd never buy a cat; there's no reason to. There's no shortage of cats around here. People dump them all the time. Having grown up on a farm, I think it's important not to domesticate a cat too much--they then have no fear, and some element of fear is necessary or they won't survive.
Catching and controlling rodent populations is the "job" of a farm cat. Farmers know that barn cats generally don't live as long as house cats, but that doesn't mean they don't feel badly when they lose one. Like most things in life, pet population should be managed. If people choose not to spay or neuter, but they provide food, shelter, and medical care for their animals, that's being responsible as far as I'm concerned.