The chant of the Propers is an integral part of the classical Roman liturgy: in fact, it was one of the first elements to be crystallized during its early development. Its position was so stable, so ‘canonized’, that it hardly developed further after the eighth century, except in one single genre, the Alleluia chant. Theologically, the selection of the chant texts (mostly from psalms) was built on a traditional biblical interpretation that can be traced to the sayings of Christ and his Apostles. This interpretation was supplied with an expanded explanation in the enarrationes of Origen and St Augustine of Hippo. Out of this tradition of interpretation a system of associations arose between the biblical and psalmic texts together with the other parts of the liturgy. To omit the chants of the Propers from any celebration of the Roman Mass – even if they are at least recited – is an inexcusable mutilation of the Roman Rite in itself.
(László Dobszay, The Restoration and Organic Development of the Roman Rite, p. 159)