Personally, I think it's an excuse to get a calf or two cheaper.. there has never been any type of proof that they don't actually do as good as one with a normal tail.
A few years back one of the beef organizations did a deal where they got some order buyers to buy a bunch of calves at the auction.. a little of everything... all different prices. They fed them all out, and posted the results. The calf that made the most $$ in the end was a rat tail. Oh.. the second highest grossing calf was earred. Go figure.