Sorry, I should clarify my last post.
Ideally you want your horse to be able to use his hind end to propel himself around the barrel. However, in younger horses, or horses who (like those that find it hard to switch leads, no matter how much you school them) are more forward going around barrels, is when the tie down comes into play, and helps the horse brace around the barrel.
I didn't mean that it is ideal for a barrel horse to brace, but the tie down is their in the event that the horse needs it.
I personally don't use the Tie down when practicing or training for barrels, and only use in in competition, as the runs are that much faster, and the horse is that much more keyed up.
I do think there is a time and place for the tie down, as is for spurs, crops, running martingales, german martingales, leverage bits, flashes etc etc.
There are many training aids that are over used/abused, and as people here say used as a crutch...however that is not to say that someone who uses one in specific situations is a bad horse person and can't ride. Of course in some instances this holds true. But when they have expelled all other options, these artificial aids can make their riding experience a more enjoyable one.
Horses are like chips...you can never just have one