I’ve always scratched my head a bit at that one: if the engines are running, then unless you are on the brakes or set the parking brake, which really amounts to the same thing, the aircraft might set off again down the runway (almost a certainty in the 777). Is that helpful while you’re trying to figure out what went wrong and what to do about it?
Our SOP is to set the PB. You can always take it off and taxi away if it becomes the best course of action.