Could be many things. Perhaps it is a vestigial structure to SOME extent? Perhaps it plays a protective role to the anus and genital region during conflict. Perhaps it serves as communication for posturing, submitting, etc. Or, both...ever notice that the submissive dog protects its genital region with its tail...perhaps to make sure that area isn't bit? A longer tail offers more protection to the genitals, but I don't put my dogs in situations that threaten their the more belly regions of genitals that are distal to the tail. Let's consider animals like deer that fight head to head (and then only seasonal) and don't need the tail for protection. Why do they have a short tail when they can "run like a deer" and change directions through the woods. They can balance just fine...darting in and through the woods full speed. Animals like cheetah, sure it plays a role but a cheetah is going much faster and also has a much larger tail. Bob cats, lynx, and such don't have one and they move fine. IMO, it isn't a big enough deal to concern myself with...tail or not...no big deal. But if one docks it I still prefer enough to cover the genitals or to serve as a handle. The bottom line is when I work dogs, I see far more influence in coordination and balance coming from other areas. Does the tail work like a counter rudder? Sure...but I personally feel that it would be like trying to alter the course of a canoe not with a paddle, not with your hand, but perhaps a finger. My point is that its role seems negligible to me...with for more influence coming from other areas that actually have "paddles," meaning areas of influence.