Most contemporary training manuals (not sheepdog related) caution against keeping litter mates. The reason for this is that you cannot give each puppy the amount of attention that is necessary to bring the puppy up as a performance dog.
I’m not going to say there is more work involved in training an agility, obedience, search and rescue, or other performance dog than there is in bringing up a herding dog, but the puppy training period is much less labor-intensive for a herding dog than it is for a performance dog.
While I use my dogs for both stock work and agility, I brought my two dogs up as performance dogs, meaning they know many behaviors that are completely unnecessary for stock work. One behavior that I taught Gel which would normally not be useful for stock work has proven to be extremely beneficial and that’s his back-up command. He’ll back up off stock on command, which is helpful when working close. Anyway, I digress here.
I know there are herding people who do a good job of bringing up their puppies, the lady I got Gel from is one such individual. There are others who are not, yet they insist that they can manage multiple dogs and still have some semblance of control over them. Not so sure I believe them.
A puppy needs a lot of one-on-one attention in order to properly bond with their handler. A puppy doesn’t need to bond with a handler in order to work stock. Some handlers use a dog’s often all-consuming desire to work as a means to overcome temperament issues that may not have been a factor if the puppy had been brought up properly. Because it is not an instinctual behavior, there must be a bond between handler and dog in order for the pair to be competitive in agility, obedience or other performance venue.
I have to wonder if the reason why some people hold on to two or more litter mates until they are of training age and then part with one or more of them if they don’t make the grade is part of our disposable society. It makes more sense to me to keep the puppy that catches your eye and put your heart and soul into that one puppy. If the breeding is there, you should end up with a good stock or performance dog.