The movies of earlier soldiers portrayed the carrying the rifle at port arms because that is what the soldiers did if not engaged in close combat. however, the movies would often show them at port arms even in close combat, wheras, in reality, the rifle would be held more ready to engage in actual combat, because the GI was actually there, scared Shiite less, and ready to pull the trigger at any sign of movement in the bush.
Low Ready sucks with a full length battle rifle even when you don't have a bayonet fixed ...
I'll post a few from WWII of Soldiers with carbines or SMGs for comparison.