|The Priest and Deacon read the Chants and Lessons at Solemn Mass
There are a number of things that occur during the Dominican Solemn Mass which originated because of convenience when one or more of the ministers had to perform some function at altar during the Foremass, and it was better for the others to retire to the sedilla (a bench with no separating arms) on the Epistle side of the sanctuary to get out of the way. This included: the procession of the subdeacon during the Gloria to the altar with the chalice and paten and their arrangement on the altar; and the arrival of the deacon to unfold the corporal during the singing of the Epistle by the subdeacon. For this reason the priest is at the sedilla from the Amen after the opening collect until he returns to the altar for the deacon's proclamation of the Gospel.
In the medieval rite the ministers recited the Ordinary and the Propers quitely. The theological logic for this was simple. The ministers are all members of the community, and the community in a religious house IS the choir. Thus the performance of those chants fell to all, including the ministers. Since their "business" often made it impossible for them to sing along with the choir, they recited the texts. There is no complex theological reason for this; and as this responsibility fell to all the ministers, not just the priest, it is not because of some special about the priestly office.
After the Prayers at the Foot of the Altar, by which time the choir (community) has finished the Officium ("Introit" in the Roman Rite) and probably gotten into the Kyrie, the priest moved to the book and the ministers arrayed themselves to his right: deacon, subdeacon, acolyte 1, acolyte 2. This is so that they can all recite the chants quietly together. The priest alone takes the cantor's parts (e.g. the verse in the Officium) and all take the choir parts. In the middle ages when the Solemn Mass was sung every day, it was expected that the ministers knew all the Propers by heart. So the subdeacon and acolytes recited along with the priest and deacon (who had the benefit of the book). Today, they just stand mute (unless they have memorized the texts).
By the time the ministers finished, it was time for the priest to go to the center to intone the Gloria. All then swung back to the Epistle side to recite it with the priest and then go sit down. The reason for this is practical. The subdeacon needed to go to the sacristy to get the chalice and paten and bring them to the altar in procession as the choir sang the Gloria. To clear space for arranging veil and vessels on the altar the priest and other minsters needed to get out of the way. At the sedilla, in the middle ages, the priest and deacon read the Responsorium (Gradual), Alleluia or Tract, and Sequence (if any) from the book held by Acolyte 1. If the subdeacon got back, he joined them in reciting. See this happening in the photo at the beginning of this post (where yours truly is deacon). They did not read the Epistle or Gospel. Why? Because it is not their office: they pertain to the offices of the subdeacon and the deacon. The ministers read these choir chants because they belong to the whole the community and they could not sing them because they will be busy preparing the chalice at the sedilla while they are sung.
After the singing of the Gospel, the priest intoned the Creed, and the procession returned from the ambo in the screen, lead by the crucifer and acolytes. They then arrived to the altar for the priest to kiss the book. As they had been in motion and the priest was waiting to kiss the book, all were busy and did not sing along with the choir. Rather, after getting to the altar, the priest, deacon, subdeacon, and acolytes swung to the Gospel side so that they could recite the Creed (already underway by the choir) together. The recitation would be finished by the time they were to swing back to the middle for the Incarnatus. After the Incarnatus, the acolytes had to escort the Cross to the sacristy, so the major ministers went to sit. Again, the recitation is (in theory) not just the priest, and the reason for not singing the Creed is practical: the ministers are busy.
The recitation of choir parts happened again at four points in the Mass. Again, it is not the priest, but all the ministers (in theory), who recite, because they will be busy during at least part of the chants. At the Sanctus and Agnus, deacon and his acolyte face the subdeacon and his acolyte (so they can keep in sync) and recite with the priest. They then get about their business as the choir sings on. For the Offertory and Communion Verse, again at these points when the ministers will be busy during the chant, so they swing to form a line to the right of the altar (Offertory) or to the left (Communion) to recite with the priest. Again, in the middle ages, all ministers recited these chants together from memory. Today, only the deacon and priest do, because only they can see the book.
So, the logic of the medieval practice is that what pertains to all (the Ordinary and Propers) is to be sung (or at least recited) by all. The Post-Tridentine period muddied these waters. About the time the Last Gospel was forced on us (ca. 1600), we also adopted a number of other Romanisms. One was that the priest recite the deacon's and subdeacon's parts (i.e. the Epistle and Gospel). The medieval logic of recitation could not explain this: it was adopted because everyone (i.e., the Roman Rite) did it. As in the Roman Rite, we also developed various ad hoc theologies to explain why we did this (well rehearsed in Mr. Kwasniewski's fine essay). This change also created a practical problem, as any Dominican who has served as priest at a Dominican Solemn Mass knows. The reading of the Epistle and Gospel, along with the intervening choir chants, takes time. The result is a rush to get them read, the chalice prepared (subdeacon) and incense and deacon blessed for the Gospel. Sometimes the choir is even finished the Alleluia before the Gospel procession leaves the sedilla!
Since 1960, however, the Post-Tridentine usage is no longer required. In the rubrical reform of 1960, it was provided that the priest (and so logically also the other ministers) need not recite the Epistle, Gospel, or other chants, "but instead may sing along with the choir." I have discussed these rubrical changes in an article in Antiphon, also available online. This is fine for the restoration of the Epistle and Gospel to their proper ministers as in the medieval rite. And the liberty to not recite but sing along would make perfect sense, if the ministers are not busy during the singing, which they mostly are in our rite. Of course, when they are busy, it is no longer required that they sing: it says "may sing" not "must sing." So, the result would logically be, mostly, for the ministers to neither sing nor recite. In practice, until the more extensive reforms of the mid-1960s, the Post-Tridentine practices just continued and the ministers recited. And the priest kept on reading the lessons. When we continued to celebrate the old liturgy after 1970 under the 1969 Rescript, this was again the practice. In fact, at every Dominican Rite Solemn Mass I have attended (in choir or as a minister) since my novitiate (1977), the priest and ministers have followed the Pre-1960 rubrics as to recitation.
What do I thnk of this? This is my opinion, and nothing more, but I think that since the major ministers and servers (usually) are part of the community, and since the Propers and Ordinary belong to the community, their recitation of these texts is very suitable. And it is permitted: the 1960 rubrical change said "need not recite" not "may not recite." On the other hand, I think the dropping of the recitation of the Epistle and Gospel by the priest makes good theological and liturgical sense (it is not his part) as well as good practical sense (it allows time for an unrushed preparation of the chalice, etc.).
But I expect that force of habit will mean that Dominican priests keep on reciting the lessons as well as the chants at Solemn Mass.