Pros: as Andrea mentioned-ability to keep the horse's attention on you and your cues. A horse PROPERLY trained AND worked w/ a properly fit, adjusted, and 'used', halter chain, should be attentive and responsive, yet quiet. This 'aid' should be: of the right length and size(the kind that 'come on' 'big' horse lead ropes are too large); it should one where the links will lay 'flat' and not be used 'twisted', it should be placed properly on the horse, so that it loosens immediately and on its own when there is no pressure on it, and it should NEVER be used to 'punish' a horse! It should be short enough so that you do not 'hold' the lead shank "ON" the chain, but on the lead shank itself. It is simply a device to remind a horse to keep its attention on you, and to learn to be light and promptly obedient in its responses. When all this is properly attended to, I don't see that there would be any 'cons'!
Margo