It is a formal guideline, at least in the United States. I happen to disagree with it, but it is found in
Hearing the Word of God, by the USCCB’s Committee on Divine Worship. The webpage I linked is undated, but I believe the document dates from 10 or 20 years ago.
It certainly dates from before the “Committee on the Liturgy” became the “Committee on Divine Worship”, but I am under the impression they are from earlier in this decade, coinciding with the new GIRM. It is also a “guideline” and not law.
“Hearing the Word of God” says, as you quoted, “Unless one is unable to hear, one should not be reading along with a text from a missal or missalette. Rather, taking our cue from the General Instruction itself, we should listen as we would if Christ himself were standing at the ambo, for in fact it is God who speaks when the Scriptures are proclaimed.”
The GIRM indeed says “all must listen with reverence to the readings from God’s word” (29). But one can listen
and read at the same time. Pope Pius XII spoke well of the practice of using a personal missal by stating (in
Mediator Dei 108) that “[m]any of the faithful are
unable to use the Roman missal even though it is written in the vernacular” (pointing out that there
were some faithful, in 1947, who were using a personal missal in the vernacular when assisting at Mass).
This was again brought up by the Church in 1958: “Those who use a
small missal, suitable to their own understanding, and
pray with priest in the very words of the Church, are
worthy of special praise.” (
De Musica Sacra 29)
I’ll reproduce all of
Mediator Dei 108 and most of
De Musica Sacra 29 here:
Many of the faithful are unable to use the Roman missal even though it is written in the vernacular; nor are all capable of understanding correctly the liturgical rites and formulas. So varied and diverse are men’s talents and characters that it is impossible for all to be moved and attracted to the same extent by community prayers, hymns and liturgical services. Moreover, the needs and inclinations of all are not the same, nor are they always constant in the same individual. Who, then, would say, on account of such a prejudice, that all these Christians cannot participate in the Mass nor share its fruits? On the contrary, they can adopt some other method which proves easier for certain people; for instance, they can lovingly meditate on the mysteries of Jesus Christ or perform other exercises of piety or recite prayers which, though they differ from the sacred rites, are still essentially in harmony with them. (
MD 108)
The first way the faithful can participate in the low Mass is for each one, on his own initiative, to pay devout attention to the more important parts of the Mass (interior participation), or by following the approved customs in various localities (exterior participation). Those who use a small missal, suitable to their own understanding, and pray with priest in the very words of the Church, are worthy of special praise. But all are not equally capable of correctly understanding the rites, and liturgical formulas; nor does everyone possess the same spiritual needs; nor do these needs remain constant in the same individual. Therefore, these people may find a more suitable or easier method of participation in the Mass when “they meditate devoutly on the mysteries of Jesus Christ, or perform other devotional exercises, and offer prayers which, though different in form from those of the sacred rites, are in essential harmony with them”.(
DMS 29)
.
