For those shooting one handed and not wearing a vest, the non-gun arm across the chest gives some protection to the heart area by possibly stopping or deflecting a bullet.
All weak hand shooting need not be predicated on the strong hand being disabled. There are many situations where the weak hand can (and should be) used to avoid exposing too much of oneself to bad weather conditions, like flying lead.
One hand practice is also good, with both hands. It is not always possible to use two hands, no matter what the great gun gurus say.
Jim