Interesting subject. Working at a vet for as long as I have now, I still have issues with this debate. I like the idea of no kill shelters, in theory they are wonderful, but reality not so much. Animals who are taken there all get to live, with the exception of horribly sick or badly injured. The animals though are often neglected because there are so many. We have a local no kill dog rescue who at any one time has around 300. We got our last dog there and he is great, but he was also a puppy and only lived there over night. Other dogs we see at the clinic that come from there may have been there for months. These animals often have fleas, worms and various illness due to living conditions. The owners of these shelters often can't pay for medical care (we actually wrote off a $2500 running balance after 2 years of no payments and they got services at a large discount from us) so injuries often go untreated.
Kill shelters are a necessary evil, but I personally wish there was more control over them. In college a few years ago I did a couple papers on the horrors of grayhound racing. Once you start researching that you get all kinds of things- not realated to greyhound racing, but to animal in general and some days I regret that I know what happens. I read about a shelter who is in a southern county in Ohio. There was 1 man who worked there. Each Friday afternoon he would gas all the animals so he didn't have to come into work on the weekend. He had no regard for them, even animals that had possible home. I also read of a shelter in a country area who would take the dogs out for target practice on the weekend.
Its such a fine line between good and bad with both types of shelters its easy to argue either side.
Bottom line Spay and Nueter your pets.