If it does go wrong
Which may not be apparent straight away. Would you want the risk of the diode pack failing some time later during a flight? I've seen electron microscope pictures of semiconductor junctions which have been subject to excessive voltage, and it's not a pretty sight! More worryingly they may continue to work seemingly O.K. only to fail completely at a later date. Ask your engineers to find out the maximum forward AND reverse voltage rating of the rectifier diodes - if they can't (or won't) then ground the main +ve terminal. With no field excitation, even residual magnetism won't induce sufficient stator current to cause overheating, but at the same time neither will there be any open circuit voltage rise...