Besides the rubrical idea that the GIRM, church documents, and traditions of the church lean strongly towards toward the singing of the propers at Mass, there’s also another important point that is missed in this discussion: important texts of the liturgy are being dropped. When the propers are not sung, it’s almost as if you’re skipping 3 short readings from the Mass (or four, if the tract is to be sung). For example, take a look at the introit for the Christmas Mass during the Night (ie, midnight Mass) that is prescribed to be sung tonight.
Ant. The Lord said unto me: You are my Son, today I have begotten you.
Vs. Why do the nations conspire, and the peoples plot in vain? The Lord said unto me…
Vs. They arise, the kings of the earth; princes plot against the LORD and his Anointed. The Lord said unto me…
Vs. Ask of me and I will give you the nations as your inheritance, and the ends of the earth as your possession. The Lord said unto me…
(continuing along with Psalm II as needed)
Translation: Gregorian Missal (ant), Revised Grail Psalter (vs)
Here’s a recording rendered wonderfully by the Westminster Cathedral Choir (this is one of my favorite chants, by the way), giving you a feeling of how well it works during a procession, in the context of the Ordinary Form:
Most parishes this Christmas will probably singing some Christmas carol like O Come All Ye Faithful. There’s nothing bad about that, per se, but it really just covers the whole “Christ is born, let’s all come worship Him” idea. Not that it’s bad, but I find it’s a little surface level for fully expressing the meaning of Christmas that the church gives us. Don’t get me wrong: it’s a wonderful carol. Carols have their place. But that place should not be replacing the music and the text the church gives us for the sacred liturgy.
On the other hand, look at the introit for this Mass. The antiphon is a rarely heard in most parishes around the world, but contains a reasonably Christmas-themed text as we may think of it: Christ is the son of God, coming to us from heaven in the incarnation. But continue to read on to the verses (the first few are above, and the first one of those is almost always used). We see a clear foretelling and reference of Christ’s passion and resurrection. We’re joyful at Christmas, but at the same time, we must keep things in perspective: this child came here to die a horrendous death for us. It’s certainly not the primary focus of the feast. But at the same time, this is the introit setting the tone for the Mass and the feast. Even in the ancient melody itself, you can hear a reserved joy, but with a touch melancholy hidden within the beautiful melody as well.
That’s just one small reason we should be singing the words of God instead of the words of men at Mass.