It is not realistic at all to say that a bunch of riflemen standing in a grassland in your country defend any better than if that grassland was in enemy territory. What, do your units know of good rocks to hide behind in their land but not in enemy land? The fact of the matter is that it is already easier to defend your territory due to city bonuses and better resupplying. Besides, "morale" does not always correlate to fighting in your own territory. Military units are trained to fight, so I don't know how "awe shucks" they would be when they weren't fighting out of their own houses.