The ethics demonstrated towards the tool suck, as do the ethics demonstrated towards the overall community, both human and animal.
If you accept responsibility for a living creature, whether you’re using it as a tool or not–whether you’re harnessing that horse to a plow or simply allowing it to wander around your pasture and eat grass, or whether you’re using that cat for rodent control or simply allowing it to sleep on the living room sofa all day–you have an ethical obligation to give it the best treatment you can. Which includes not only decent food, clean water, and a sheltered place to sleep, but also all necessary medical care, including immunizations and visits from the vet or farrier when necessary.
Also, someone who doesn’t see to it that the animals for whom he accepts responsibility receive proper immunizations is contributing to the overall animal disease problem in the community (rabies, distemper, parvo, feline leukemia, equine encephalitis, etc.). There are also infectious conditions like mange, ringworm, and various intestinal parasites that can be spread by animals who are allowed to run loose but who receive no veterinary care. I would wonder how his neighbors feel about his contributions towards maintaining the disease reservoirs in his community.
And if the monetary expense of the creature’s upkeep bothers him, then he shouldn’t accept responsibility for it in the first place.
We’re familiar with barn cats. We don’t agree that it’s okay to let them run around unimmunized and unneutered, and to shrug it off by saying, “They earn their keep, and it’s a kind of symbiosis, and anyway the coyotes keep them in check.”
If you feed them, they’re yours, and you look after them the same way you’d look after any other pet or farm animal, whether it was earning its keep or not. If a friend of yours “didn’t have room” for that potbellied pig anymore, or that elderly pony, and you said you’d take it, you’d see to it that it had its shots, and so should the barn cats.