PfcAJ
Contributor
Having the inflator on the right post gives you something else to breathe off, in the unlikely event you have donated the long hose then roll off the left post in a restriction so tight you can not turn it back on again.
I suspect this has never actually happened in the history of diving though!
A far more common scenario is an inflator that sticks. Going vertical allows you to dump your wing with your left hand, simultaneously dumping your suit in the process. The right hand is free to attempt to disconnect the hose or shut down the right post if that's not working out (frozen, stuck, or just butterfingers).