Now I wouldn't usually suggest this but my horse is a little long-backed and struggles sometimes to get her hind end underneath her but can ride like a bull sometimes now that she knows how to push into the bridle (she gives me the push I feel when riding upper-level full-sized horses). So I feel like it would help her progression and education without teaching her she can run through her rubber-covered baucher.
Part of the reason I'm coming to this conclusion is that I use a 3-ring elevator for jumping and when I work her in that she seems to understand everything I ask her and is capable of giving me things like an FEI-quality medium trot. She seems to understand how much she needs to come up underneath herself and how up she needs to be in front but I don't want to overuse that bit because I need it for jumping and x-c.
Now, before someone jumps down my throat about taking the "easy route". I do dressage 4-5 days a week with my pony, she's perfectly capable of getting in a frame and accepting the bit. She does a lot of lateral work including: walking & trotting shoulder-in, walking & trotting half pass (has shown some aptitude for the canter half pass), half and full walking pirouettes, she has had some trouble with the travers (haunches-in) but can do them at the walk and is starting to be able to do them at the trot (this I somewhat attribute to her not exactly knowing where her hind end should be because she will do them in the elevator), she can leg yield at the walk and trot, and I can even do fun things like halfpass to X and then leg yield to the rail so she's pretty adjustable through the body. I've put in lots of dressage hours on this horse.
I just feel like she needs the extra support to improve her balance and adjustability at the canter because she's got so much push into the bridle it's hard to collect it all if she get's confused and starts to rush and brace.
I have ridden in a double bridle before, I know how careful you have to be. I've spent a lot of time sitting around stumped about this decision because I don't want to overbit her and have her go back to being extremely backed off but I think this might really be helpful if used in moderation as a way of assisting her normal snaffle bridle training.