I can appreciate leaving a dog intact if/when it is owned by a breeder. After all, they are equipped to keep their animals separated during mating season unless they are specifically pairing two lines. However, I personally believe that pets should be nuetered and spayed.
First, because breeders take care to make sure "accidents" don't happen. And an intact male in the presence of a bitch in season is going to do everything he can to "complete his mission!" It doesn't matter if she's the same breed, smaller, larger, cute, a good match, or comes from the wrong side of the tracks. It only matters that she's in season. As a dog park regular, most of us roll our eyes when someone brings in an intact male. I'll let you guess how most of us feel when someone shows up with a bitch in season!
And that's the second issue. Most "dog owners" don't have a clue about what they should look for, or how they can tell if their bitch is in season. They don't know how long it lasts. Or what phases are involved. Or anything else about how to best handle the situation. And many of them really don't care. They think it's cute that their bitch gets knocked up, "it's a great way for the kids to learn" mentality.
Combine the first two points and you can begin to understand where Basenji mixes come from. It is my understanding that Basenji breeders frown on diluting the breed.
The third point is how dogs (especially in a dog park environment) behave around a bitch in heat. Any given dog can go from mildly agressive to murderous in seconds if they believe they will not be the winning suitor of a bitch in heat.
I was unaware when my Doberman bitch went into season (several decades ago). Two dogs followed her home and stood on the stoop (at the door) waiting for me to let her come back out to "play"... In a split second, much to my horror, the larger of the dogs lowered his head and broke the neck of the smaller contender. There was no aggression. No warning growl. No blood. No "I'll fight you for her." Just "survival of the fittest" instincts. (All of my bitches have gotten spayed since then!)
So, in summary: it makes sense for breeders to keep their dogs intact, not necessarily the "average" dog owner. Make the best decision for your individual situation, but try to think through all the potential outcomes.