Longhorn meat is lean. And lean beef is becoming more popular. But since there is usually not much meat on longhorns that do go to the sale barn plus the meat there, because it is so lean, does not marble well. Add in the horns, which can cause bruising, and the fact that they arent solid colored, or atleast mostly solid colored like herefords or baldies, and those are other reasons they are docked. Also, they are docked because they can be.
I know many people that have done well selling their calves, before or after harvest, to people searching for lean meat. More niche market type of sales. That is one of the great things about Longhorns, there are many many different avenues of selling them (show, ropers, lean beef, etc...)
Hope this helps some.
Ryan