well its like this. The shanks is for the comfort of the rider. If you are racking right along for a mile or two, or off and on all day. Its much easier for you the rider to only use a pound or two of pressure on the bit to keep them going at the desired speed and gait. W/o the shanks you have to use alot more pressure to get the same effect on the end of the bit.
Curb bits are leverage bits that increase the pressure exponentially. You might not think that "a pound or two" of pressure is all that much, but if you actually take the mathematics of the bit and pressure ratio into account you are putting a lot of pressure on that horse's mouth.
Let's say that you're using a bit with an 8 inch shank (which, from what I've seen is about midpoint in what's available in "gaited" bits) and you're maintaining 2 pounds of pressure on the reins and occasionally using 4 or 5 to cue for a downward transition or a stop.
Doesn't seem like much, right?
But those shanks are multiplying the pressure so essentially, you are maintaining 16 pounds of pressure on that horse's mouth. When you ask for him to slow down or stop, he's feeling 32-40 pounds of pressure in his mouth.
When you think of it like that, it doesn't seem like "not that much" anymore, does it?
That's the exact reason why shanked bits like that are supposed to be ridden on loose reins. Picking the slack up out of the reins on a bit like that feels, to the horse, like several ounces of pressure from a regular snaffle.
And, as I said, if you're having to pull your arms out of their sockets, even in a simple snaffle, to get your horse to maintain his gait, then there is something seriously wrong with his training or the way he's being ridden. It shouldn't matter if he's a speed racking horse or a Percheron or an old plodding grade horse, a hard mouth is still a hard mouth and training is still training.
I suppose folks who are happy to put their horse's mouth into a vise grip every time they ride...well, to each their own, but folks like that will never lay a hand on any of my horses.