Jeffrey Tucker of the New Liturgical Movement came across a reference to a decision of the Consilium (whose job it was to put Sacrosanctum Concilium into effect) regarding singing hymns instead of the propers of the Mass (i.e. the Introit, Offertory, and Communion antiphons):
What must be sung is the Mass, its Ordinary and Proper, not “something”, no matter how consistent, that is imposed on the Mass. Because the liturgical service is one, it has only one countenance, one motif, one voice, the voice of the Church. To continue to replace the texts of the Mass being celebrated with motets that are reverent and devout, yet out of keeping with the Mass of the day amounts to continuing an unacceptable ambiguity: it is to cheat the people. Liturgical song involves not mere melody, but words, text, thought and the sentiments that the poetry and music contain. Thus texts must be those of the Mass, not others, and singing means singing the Mass not just singing during Mass. (1969, Notitiae 5, p. 406)
While the GIRM does allow for hymns to replace the propers, one has to wonder why this permission for substitution came about, since it goes against the liturgical tradition and, as is seen from the Notitiae quote, the former legislation of the Church.