Commando Brado, wow this was a bad predicament to be in. I’m assuming that Japan was in allied hands and this was a last ditch effort to get it back (it gets one turn to do so). Boy did you get screwed by not having any ships that could take out those US subs, or force them to submerge. I didn’t think much of the a/c not having an attack value any more til now.
Edit: The only other scenario I could see where you would want to bring Jap transports into sz 6 combat is you still have Japan, and are trying to liberate Korea.