Obviously there is a lack of belief in the Real Presence behind such a position, among other errors.
So explaining our faith and its scriptural basis to those already attending holy Mass regularly on Sundays is in fact not "preaching to the choir" but rather giving our people exactly what they need to live their faith every day.
In a Bible-only world, where many have refashioned the faith to fit their own designs, we must explain the practice of the Church to worship on the Lord's Day. The faith started in the Church, preached and proclaimed among the people of God before it was fully set down in writing as the bible we know today.
In the Scriptures for Ascension we see how the people reacted after Christ ascended to the right hand of the Father: they went to church together, although they used different words from their Jewsih experience to describe it.
Christians, as we see from Luke chapter 24, began to worship together from the beginning: "and they were continually in the temple praising God."
Now, we know from the Commandments that it is not required that we be in the temples of our churches "continually"; only Sunday worship through Mass is absolutely required to maintain living contact with the resurrected and ascended Jesus Christ and to "keep the Lord's Day holy" as commanded by God. To be scriptural, to follow the teaching found in the Bible, we must attend Sunday Mass with the same body of believers connected through the Church today to those very first Christians.
We take part in the mission of the universal Church today, also explained in Luke chapter 24, every time we celebrate holy Mass with the Lord, interceding for us at the right hand of the Father, and his one people united in the Holy Spirit:
"repentance, for the forgiveness of sins, would be preached in his name to all the nations."