You can trust a new owner with your dog, intact or not, or you would not sell them the dog.
But, circumstances change. In 12 months time that new owner might have lost his job, or had a crippling accident, or even died. It may not be up to him to say where the dogs should go. Yes, yes we should all have a will that clearly states what should happen to each of our critters, but things change, sometimes the person who had agreed to take a dog is now in no position to do so.
Working lines are not immune to people who are going to use a dog foully in their quest to make money. If someone's requirement in taking a dog is that it is and remains intact, then they most likely want to breed the dog. That in and of itself isn't the end of the world, so long as you can carefully pick the people who take the dog. But, if its someone's heir that now needs to get rid of the dogs, and the only thing that person knows about the dogs is that they were bred, or were going to be used for breeding, they may look for whoever will give them a decent amount to take the dog.
And then the dog can fall into the hands of people who will sell the puppies of a dog with accomplishments and a working line pedigree. By this site alone, I think we can agree that the working lines are popular. That means people will snatch up these dogs and some of the people doing the snatching will be people who should not own a living creature.
By altering your dog before rehoming it, the only value you remove is reproduction. This means that the foulest of owners will not EVER agree to own your dog. Those owners who have breeding as a part of the plan they have for the dogs that they prize highest, to pass on the genes, will also pass on the dog.
I think you can look at in with that four square way: + + ; + - ; - + ; - -.
++ If you keep the dog intact and it gets the best possible situation, is worked with and eventually responsibly bred and passes on its excellent genes to its progeny.
+ - If you alter the dog, and it gets into the best possible situation, is worked and cared for properly, turns out to be awesome, but it can never produce itself and pass on its genes.
- + If you alter the dog, and it gets into a negative situation, the chances are that the worst kind of owners will not want the dog as it is already altered, and the possibilities for neglect and abuse of the dog is limited to that dog alone.
- - If you do not alter the dog, and it gets into the worst possible situation, it is kept by someone with dozens of dogs, in small pens, pumping out puppies.
Then you have to look at the likelihood of the dog being passed into other hands. Is the certainty of your dog not having the + + situation worth the risk of the dog landing in the - - situation. That is just a question that the original owner has to come to terms with.