I have a friend who rides Hunters and uses this on her trainer's advice, just as an every once in awhile thing to get her mare to stretch and relax, so I don't see it as a negative thing.
***I will say, you should have trainer or someone else very knowledgeable on the subject teach you how to use it, because the results can be disastrous if you don't, as with any training tool!***
I personally lounge my mare in side reins set sort of the same way the neck stretcher is (except I don't ride her with them on, only on the lounge line), to encourage her to lower her head and move "long and low" to build up her topline. It's just used every once in awhile as a training tool and I have had good success that then transfers to under saddle with her. Here's what I do with my mare with the side reins (they have elastic on the ends near the bit by the way, so that they give a lot)...I just use them every now and again to encourage her to stretch, as she tends to just throw her nose up in the air and hollow out her back, causing all sorts of fun problems