They're a good training aid if used by someone who actually understands how to use them, and on a horse that genuinely requires a little extra assistance to soften it's jaw and drop it's head, as opposed to the many people that put as many different gadgets on their horse as possible because it 'looks cool' to have multiple pieces of tack on a horse at once.
I used draw reins for 4 rides on my old QH x appy mare. She was extremely downhill, hung onto the reins and stuck her nose out.
All draw reins do, if used correctly, is to allow the rider to exert a little downward pressure on the bit should the horse throw its head up or hang onto the reins, using a small amout of contact on a second pair of reins, much like riding in a double - there is no contact on the curb unless the riders choses to take up a contact on the second set of reins - so that there is no pressure exerted on the horses head from the draw reins whatsoever until the rider choses to put pressure on. The pressure is simply an annoyance, that makes life uncomfortable for the horse to have it's head up, and comfortable for it to soften the jaw and lower it's head.
If the rider knows what they're doing, it is really up to the horse whether there is pressure or not.
It took my mare only 2 rides in draw reins to understand that she was the one controlling the pressure, then after that I rode in them only once a week for 3 weeks to put in a little reminder. After that we had no dramas at all with head carriage. I also think that the added help of draw reins for a few rides allows a rider to focus more on the hind legs and driving them, as lets face it, it IS tempting to think more about head set than hind legs because you can SEE the head when you're riding.
Although in saying this and being FOR the use of draw reins with an experienced rider, they should not be used on horses that tend to back away form the contact and won't come off the leg, as draw reins will further enhance this, they may make the horse's head and neck look beautifull and 'round' but the hind legs are doing nothing and the horse is on the forehand, achieving nothing.