The shank is for the rider of course, so it depends on the rider. The mouthpiece is for the horse. Puzzled? (Please check out more articles on my website about the new understanding of bits - Myler Bitting System Philosophy.) Read the large letters below for the key to this whole thing about using a shanked bit.
When you move beyond the very basic training into body control training like I do with my pre-reining, similiar to dressage training, we require the horse to do more lateral work (reining and western, of course wants more lateral than dressage. Dressage does a lot more straight work then western relatively speaking) then you need a longer shank to quiet your hands down.
Think of using a shank instead of nosebands like a flash, etc. Or tie downs used in western. Think of the shank on a bit as giving your hands more authority without more pressure. It doesn't hurt the horse because the mouthpiece hasn't changed.
A snaffle was meant for training (as it's designed to have a direct pull on the horse's mouth to better position a horse) so when the horse is finished to whatever you like, then a shank is the key to keeping them soft and more particularly, to keep your hands light.
The shank helps those riders whose skill level is not advanced to keep their hands light. A rider doesn't want to learn accidently to be heavy handed.
Of course, rough hands should stay with shorter shanks or no shanks (ie. a snaffle). The problem is that a snaffle, if over used, can cause someone to be rough or stay rough. Not a good thing.
When you start back into a horse's training or yours after a timeoff period, you will need the snaffle until your hands are light again. As long as it's effective, of course. If the horse is so dull in responding to your reins that the horse ends up teaching you to be rough, then DON'T DO IT. Please go to a curb and shanks. There are many shanks to suit all sizes of people. :)
Of course, if a rider's hands are SOooooo skilled then they don't need anything at all... do they? :)
In my training, I start with a snaffle as I have lots of training to do - I have to move my hands a lot to position a horse and teach them their cues. Once the horse starts to understand their job and I'm starting to just ask for things instead of assisting, then I move into a bit with shanks.
I feel it's important to reward the horse for learning so I go into a curb as soon as I can. So the length of the shank depends on the horse's personality (Dale Myler says this is the most important aspect to consider) and my skill level.
Of course, for the client's horse, I use the bit that I would recommend for the rider too.
I hope that helps. Please contact me if you need help with deciding on a bit to use or to understand more about bits.
Putting my spin on bits!