It is a wonderful thing to adopt a dog from the shelter. My dogs are shelter dogs, and always will be.
However, folks who are looking for a specific breed for a specific purpose will only find this with a reputable breeder. There are also people who need to have the guarantee that the pup they purchased will have certain traits and temperament. Many shelter dogs are just a crap shoot.
For instance, if I wanted my ACD to be a guaranteed herder with a great temperament, I would certainly not be looking in a shelter. People who breed for this kind of quality would make sure none of their dogs ever ended up there.
As far as anxiety, that can be inherited, and a shelter pup is certainly capable of having this problem right from the beginning. As a matter of fact, I just had a customer in the other day with a 7 week old Huskie mix that is already exhibiting the symptoms. He is ill-equipped to deal with something like this, and may not be able t keep the pup. If he had purchased what he wantedfrom a good breeder, he would have no cause for this kind of heartache.
Actaully, if more people bought their pups from good breeders, the shelters would not be as full as they are. The problem is not these people, it is the people who buy from pet stores, and bybs, then dump their dogs, because there is no guarantee, and no breeding standards. The problem is these breeders who don't care where the pups they produce end up as long as the money changes hands.