I don't see them as a bad thing unless they are used wrong
. Most don't understand the use for them and just slap them on their horse because they complete the look or to cover up traning issues. I'm on the fence about the whole balance issue and bracing with TDs in general. My horses run without them.
The only time I ever used one was when Charlie used to throw his head ALL the time. He could literally break my nose if he wanted to, I got countless bloody noses. Used the tie down for a few rides, realized it was wrong and fixed his holes the right way. That was when I first started riding and I don't really like to use them now, but I would if it really helped one of my horses run.
The horse should run fine without though, and have no holes in training before a TD is put on. They may run better with it, fine. I just hate when they're used as a cover up. Seen one to many horses explode because the TD was cranked tight and they were NOT ready to be running at shows, let alone in general.