There is an arguable benefit to a managed colony. Cats are territorial, so they tend to protect their area and their food source. When I started doing TNR a few years ago, we had animal control and some neighbors trapping our numerous feral cats and having them euthanized. The sum result? More cats, year after year, and none of those cats were spayed, neutered, or vaccinated. In the spring, I'd see as many as 8 cats at a time in our parking lot. They'd keep breeding, and the part-time trappers were only maintaining the numbers -- not really putting a dent in the population because the remaining cats would keep breeding.
We started doing TNR on feral cats. As concerned volunteers, we were religious about it. About 40% went to adoptions. Another 40% were spayed, vaccinated and released. And about 20% either disappeared on their own or had to be put to sleep because of disease, etc...
Now, from about 30 cats and kittens, we're down to about 4-6 "regulars" in one colony. In the past year, I've seen one or two traveling strays, but they come and go. In our direct colony, we've seen no new kittens. In neighboring colonies, we've seen only a few kittens in two years.
That was a DRAMATIC decrease from a few years before. I've watched these remaining cats chase off new cats. If we removed these vaccinated, spayed ferals, the other cats -- unspayed and unvaccinated -- would probably move in.
So which is better? Having a small number of spayed/neutered and vaccinated cats in a managed colony, or remove those and have unspayed, unvaccinated, unneutered cats move in?
Reason Two: our TNR and animal rescue groups are made up of all volunteers. They're responsible for spaying/neutering, managing, and controlling hundreds of animals every year. If other people are having the animals killed, then what's the incentive for the TNR groups to continue with their work? The truth is that the volunteers are much more effective at maintaining and reducing populations than part-time animal control operations. And they often pay for their efforts with donations, rather than community taxes.
Reason Three: Most people like animals, or will tolerate them. When doing TNR, you're much more likely to get homeowners to cooperate with you (and let you trap on their property) if you tell them you're having the cats spayed, neutered and vaccinated. If people think the animals will be killed, they're less likely to help -- so those cats keep breeding. Another reason why TNR works better than "kill" control.