The words, "In addition to all of these" have been translated in other versions, "Above all...". The idea here is to recognize that taking the shield of faith should not to forgotten. The other apparel of the armor is worn on the body. This particular object, the shield, is something that is not worn, but is held.
The important truth here is to recognize how important FAITH is to a believer facing the battles against evil. Its great to have Truth, God's Righteousness, and Peace in our lives, but we must not venture forth against the enemy without a firm hold on Faith! From the very beginning of the Christian life the believer enters by faith and needs to continue to walk by faith. Note the words of Colossians 2:6 NLT - "And now, just as you accepted Jesus Christ as your Lord, you must continue to follow Him." You entered Christianity by faith and it is by faith you live. Faith believes God! Get that? FAITH BELIEVES GOD! As I like to say, "God wants to be believed."
Now, faith comes from hearing the Word of God, Romans 10:17. This "hearing" involves paying attention to the Scriptures whether you listen or read. Most believers in the New Testament times didn't have a copy of the Scriptures to read. They had to listen and remember what they heard. Most of us today have God's Word, but it doesn't do much good unless we take time to read it! Spending time in the Scriptures gives the believer the opportunity to read how the people of God trusted Him and how He proved Himself over and over again by doing what He said.
Holding the shield of faith keeps the promises of God's Word always in our minds and hearts. So when the devil throws his "fiery arrows" of lies and deceits at us we refer back to what God said and, thus, avoid the penetration of the arrows hitting our hearts and minds. Satan hates our faith - he is always defeated by a believing Christian!