Maybe this is why most people use them incorrectly
Their purpose is to force the horse to "give" to the reins. In correct dressage the horse should NOT "give" to the reins.
What is being described isn't the purpose of drawreins. Draw reins may be used by an experienced rider to help develop proper flexion. They are used by riders at the SRS, the mecca of classical dressage. If I remember correctly, originally they weren't supposed to be used through the bit, though that is how you see them configured most commonly. By applying pressure with the inside draw rein and giving with the outside, the horse may be encouraged to soften at the poll and in the jaw. Draw reins should not be used on both sides at the same time. To do so creates a pulling match between the rider and the horse's poll. Many people think they can be used to force a horse on the bit or into a frame, but it doesn't work and is not their intended purpose.
Personally, I have seen more harm than good come from their use and I don't like to use them myself. But in the hands of knowledgable users, they aren't a problem. The problem is not with them as a tool. It is with the people who think they know what they are for and how to use them.
lstevenson, you say a horse shouldn't "give" to the reins or the bit. What is your expectation of what a horse should do in response to an aid from the hand?
lstevenson, have you read this?
Well, you haven't worked with all of them. Here is a link to an article by Karl Mikolka, former Chief Rider from the SRS. I'm sure if I got his title wrong, someone will correct me! Someone already mentioned this -- but I submit it here for your convenience, lstevenson:
I agree completely with you that true lightness comes from riding the horse rear to front, and all that jazz. I still contend that a horse can be ridden correctly forward into contact, accepting the bit (as opposed to "giving", perhaps we are now getting caught in a semantical net), coming round , engaging his hindquarters, lifting his belly, all the stuff that goes into making fine dressage -- but that some may need temporary help with drawreins.
The horse I described in my earlier posting is now performing (in hand) piaffe and beginning passage. She can lower and carry with her hindquarters like mad -- but is not yet strong enough to carry a rider and perform the same feats. She can -- but for a few steps at a time only. And believe me, she NEVER gives me a false sense of "lightness"! Draw reins did her no harm, and even helped us get where we are now. She is a difficult, intelligent, sensitive creature who was not well served by her early riding. She also has a somewhat low-set neck and a tendency to overuse her underneck -- similar to my friend's horse at the BNT's clinic. She frustrated the other professionals I worked with -- not just me -- and these are people with extensive talents and knowledge. Training her is a journey of discovery -- I've learned more about my strengths and weaknesses as a human being and as a rider through her. It has been humbling, enlightening, frustrating, and challenging. She has made me a better rider and teacher to my students. I owe this mare a lot!
So don't condemn the use until you've met that one incredibly difficult, stubborn, not-quite-perfect individual who challenges everything you believe and everything you think you know!