The problem is people use Tie downs because they are ignorant in the ACTUAL purpose of the tie down is.
The true reason for a tie down, is for performance horses (ie barrels, roping) need the tie down to brace against when running patterns.
But alot of people don't know this so, figure hey my horse throws its head, he needs a tie down.
Tie downs do have a purpose so its not a bad piece of equioment, its how its used that is the problem
My barrel horse has no head issues, you could run her in a snaffle with nothing else, but when I want her to run good I do put a tie down on her to help her balance. This way when they're rating they have something to push against to slow down and turn around the barrel. When some horses go so fast it is hard for them to rate themselves off of their own balance, but tie downs really help your horses balance. Most horses, though, it just depends on the horses needs. They all work so differently and need help in different things.