Breeds such as Aussies and Border Collies express themselves strongly with their beautiful fanning tails.

If there was -ever- a valid reason such as tail degloving, there is now, no valid reason.  Now it is unnecessarily done only for cosmedic conformation to breed appearance standards.

The below is quoted from Wikipedia

The American Veterinary Medical Association (the largest veterinary professional organization in the United States), disputes these justifications, saying "These justifications for docking working dogs' tails lack substantial scientific support. In the largest study to date on tail injuries in dogs the incidence was 0.23% and it was calculated that approximately 500 dogs need to be docked to prevent one tail injury."

Modern practice
Docking of puppies younger than 10 to 14 days old is routinely carried out by both breeders and veterinarians without anesthesia.

[...]

A study conducted at the University of Queensland Companion Animal Veterinary Hospital found that puppies display signs of pain when tails are docked. Of the 50 puppies observed in the study, all 50 vocalized (shrieked) when tails were amputated, averaging 24 shrieks per puppy. The American Veterinary Medical Association has expressed a concern that pain during this early development period could cause lasting damage: "There is evidence in many species that noxious stimuli in the perinatal period may permanently alter the normal development of the central nervous system and have negative long-term consequences."

Criticism
Robert Wansborough argued in a 1996 paper that docking tails (very short) puts dogs at a disadvantage in several ways. First, dogs use their tails to communicate with other dogs (and with people); a dog without a tail might be significantly handicapped in conveying fear, caution, aggression, playfulness, and so on. Leaver and Reimchen, in 2007, found that longer tails were more effective than shorter tails at "conveying different intraspecific cues, such as those provided by tail motion".

It has also been suggested that certain breeds use their tails as rudders when swimming, and possibly for balance when running; so active dogs with docked tails might be at a disadvantage compared to their tailed peers. Videos comparing docked and undocked dogs running and jumping show that dogs who are docked have to work harder to compensate for the loss of the tail. Canine pathologist and sports-medicine expert Prof. Chris Zink believes the extra stress imposed on the joints can have long-term health consequences.

In 2007, Stephen Leaver, a graduate student at the University of Victoria, published a paper on tail docking which found that tail length was important in the transmission of social cues. The study found that dogs with shorter tails (docked tails) would be approached with caution, as if the approaching dog was unsure of the emotional state of the docked dog. The study goes on to suggest that dogs with docked tails may grow up to be more aggressive. The reasoning postulated by Tom Reimchen, UVic Biologist and supervisor of the study, was that dogs who grew up without being able to efficiently transmit social cues would grow up to be more anti-social and thus more aggressive.

[...]

Docking has been condemned by the American Veterinary Medical Association, the American Animal Hospital Association,  and the Canadian Veterinary Medical Association. These organizations have also called on breed organizations to remove docking from all breed standards.