I personally don't like draw reins because of the way i've seen them used. however, a benefit is that if your main set of reins breaks, you have another. in polo, you have to have draw reins, i have seen people's main set of reins break during a match
I think draw reins work wonders if used correctly and not abused. They should be a tool, as mentioned in the last paragraph, and nothing more. And certainly not on a permanent basis.