It's one of the "NH" ideas, and I have seen SO much harm done by NH in inexperienced hands it is just not true, but there you go.......
A snaffle is not a gentle bit.
It was never devised to be a gentle bit and dressage horses, if they are any good, go through that stage very quickly.
I would never school a horse in one half of a double bridle...why would I do that???
I was happy to ride one of my Arabs in a noseband (just a drop with the reins attached, no pulleys or gimmicks) but she seemed to have an aversion to a bit, which took forever to overcome, so an interim methods was needed.
She did eventually accept a bar snaffle, and this is the bit I prefer if I have to use a snaffle for any reason.
I also take my driving horse right up to being put in the cart, in just a halter, and bit them separately.
You would not be allowed to drive in the ring without a bit, I am pretty sure.
We did use a bar snaffle on One Day Events with the Pony Club but in the end found it easier to just accept the penalties for using a double for the dressage, as a double is what is, basically, correct for dressage, and what got the bast results.
It is very difficult to emulate the work of a curb bit with a snaffle, and totally pointless as that is exactly why the curb was invented...because it was needed.
Anyway, a bit does not work on pain, and it does not work, as the Dr Cook and the Bosal do, on pressure points...wrongly adjust those and you really could cause pain, a badly adjusted bit just causes annoyance!!
My advice to you is to get the animal used to a bit, even if you decide to work it without one, you never know what life will bring, and they may not be with you for all their lives.