I dont know but around here the highest selling calves are long and black and the next highest are long gray calves. most everyone uses either angus, brangus, or charolais bulls. The rat tails do get discounted but not the regular charolaisxangus.
We kept two heifers as an experiment to see if the rat tails were really inferior or if it was just a reason to dock calves at the sale b/c they could. both of these heifers were great quality aside from the rat tail. One is now an 8 year old cow that has had average calves, either black, light gray, or red. The other is a two year old heifer that had a heifer that is the dark rat tail gray but doesnt have a rat tail. i personally think they are just docked b/c they can be. the only thing i can maybe see a rattail being less efficient at is swatting flies.