Oh I disgree! If the horse has a melt down when it hits the pressure of the bit with the side reins, there is no redemption. I've trained horses I would classify as claustrophobic when it comes to sudden pressure.
Side reins are a short cut.
You keep them long so they arent forced into a frame, but if they try to fling their head around they will feel it, this way if they try it once you get on they wont flip out with you on their back.
I don't see how its a short cut, its a training tool so your horse doesnt have to be hit with 10000000 new things the first time you ride