Well, I think the rest have said it best. I think the chances of having a more balanced dog may be with reputable breeders. A dog that is not balanced, which one, my last GSD was one of the best dogs I've had. He was scared of the oddest things, but never mean in any way or fear aggressive. While my newest installment, is in a class by himself. Both from BYB's. The jury is still out on Cruz as he's only 8 months. I paid $450 for him.
But I think there are good BYB's out there if you look for them. But I see nothing wrong with buying from a reputable breeder either. But I think if you buy 10 dogs from each, I think your percentages are better for a balanced dog with the reputable breeders than BYB's. Reputable breeders place and match pups to thier owners, they also do other things that BYB's do not do in the way of sensitizing pups. It's not a situation where you show up, dish out the cash and drive off with the pup of your choice. So I think reputable breeders have that over BYB's. Not to mention being more careful in selection of breedings.
I think alot of people still enjoy showing up and picking out thier dog. I think it's also the money as alot of people just won't pay alot for a dog either. I also think the biggest is they just don't take temperment into the equation when looking for a dog.
They know what they want, find one on craigslist or the local paper and go pick him/her out.