I have seen waaay too many people with unaltered females, who have been impregnated multiple times do to "accidents". I think that is crap. Either you spay/neuter or you watch your dog 24/7. If you don't do either of these things, you are
attributing to over-population, period.
I think it's kind of unpractical to say that only people who are super-irresponsible will allow their unaltered dogs to breed... when my Mom was younger, they had a Chihuahua bitch who was very young and going through a silent heat very pre-maturely. This was in the 70's, when spaying & neutering was not a common practice. During a short potty break in the backyard (which, I'm sure all of us allow our pups outside for a while) the nieghbors Lab jumped the fence and bred her, somehow, and they had no idea until it was too late. She died. Because most people live within close proximity of other people's dogs, especially in the suburbs, and have yards that they allow their dogs to roam often, it is not safe to let an unaltered dog outside alone. For instance, my neighbors had a Doberman girl who was not spayed, and she would often dig to get into my backyard when Gonzo was outside because they loved each other (we have a doggy door, so Gonzo does go outside for short periods alone). If he wasn't neutered, that could've meant an unplanned pregnancy easily!
I also think it's pointless, and stressful, to put a dog through the urge to breed if you are not a responsible breeder. Having bunnies really opened my mind up to this! My two boys hated other bunnies, were horrible indoors with marking, and would literally jump the bones of every blanket/stuffed animal/leg in sight!! After they were neutered, allll of these behaviors went away and they were calm & sweet for the rest of their lives. And so much happier! I've experienced the same affect after spaying & neutering dogs AND cats. We had a Himalayan female who, before she was spayed, would literally sit at the door all day wanting to go outside so she could breed! After spaying her, she was a totally content & happy cat. I have only seen the good affects of spaying & neutering. I have also seen unaltered dogs go absolutely ape-shvt! Like, a really intelligent lady brought her in-heat BC to Flyball practice 3 weeks in a row... all of the males on the team are neutered, except for one show/working Dobie. The neutered males could care less about her. The Dobie could not even do recalls, and was a complete wreck the whole time! Like, foaming at the mouth, acting erratic, barking, trying to hump my dog
... it is not pretty.
But honestly, if you consider yourself responsible enough to watch your dog at all times, never allow it in a situation where ANY strange dog could come in contact with them without supervision, and you're happy dealing with the marking & neurotic behaviors of unaltered dogs - that is awesome! However, NO ONE is perfect and one moment of laziness and just letting your unaltered dog into the backyard for a few minutes alone can easily cause an "oops" pregnancy.