They have their place, they were designed with the right intention in mind. Unfortunately they fall into inexperienced or impatient hands desiring a 'quick fix' to give their horse a 'pretty head set' and there fore get a bad name. The biggest negative effect of them is that they very easily teach a horse to back off the contact by ducking behind the vertical and not taking the rein. If you just want your horse to 'look pretty' with a tucked in head to show your non-horsey friends, then by all means, haul on a pair of draw reins and you'll have your desired result. But if you want a horse that is travelling correctly with a connection from hind to front, teaching them to suck back is going to set your training back hugely, it is much harder to teach a horse that sucks back to step forward and take the contact, than it is to teach a horse that braces and leans on the bit to soften and accept the contact.
I have used draw reins on only a select few horses. One an ottb that was so nervous about relaxing his neck that through 'traditional' training it would have taken me months to build his confidence that he did not need to be held up by the rider, that using draw reins for 2 rides gave him the added encouragement to release his neck and lower it. Two rides only, so that I knew he'd worked out for himself that he wasn't going to fall over if he relaxed and lowered his neck, and I was able to continue training as usual.
I would only ever use them on a horse like this one, that wasn't sure that it COULD reach down and had little likelihood of avoiding the contact by sucking back. At the first signs that the horse is ducking off the contact, I remove the draw reins and ride them 'pony club style' allowing them to stretch forward and out making them take a contact, until they can start to come back into a more appropriate frame without coming behind the vertical.
I don't believe it is fair to completely disregard draw reins as a training tool, as I mentioned initially, they have their purpose and are simply meant to assist in training, not be the main training tool. In the wrong hands they cause more harm than good, and should be left to only experienced riders who have a very good understanding of a correct contact in the bridle and great knowledge in how to employ the use of draw reins only when absolutely needed - otherwise they should hang slack and not have any influence on the horse what so ever.