I see no advantage in passing. If your units are in a good defensive position (good terrain/supported) that you don't want to move then at least fire your cannon. If you get a single hit, then that is one less enemy block that you have to worry about later and that is one less die your enemy gets to roll if/when he attacks with that unit.
I have played hundreds of these scenarios. There have been only a very few times when I didn't have a good move, so in those rare occasions I just throw away my worst card, ordered my units to do nothing, and draw a new card; turn over.
Usually when you realize you are losing a scenario you will feel that you are just one move behind your opponent.