IGMR (editio tertia) para 85, citing documents of 1967 and 1973, describes it as valde optandum that the faithful should be communicated with hosts consecrated in the same Mass. The same hope had already been expressed by Pius XII in Mediator Dei (see footnotes below). As far as my experience goes, this 'hope' is widely disregarded, both in the more traditionalist churches and in extremely untraditionalist Catholic churches, for reasons of manifest plain practical convenience; by communicating the laity from the Tabernacle, one avoids having to guess at the number of communicants present; a priest does not have to stand at the altar before the ablutions chewing away at what Anglicans used to call the Remains of the Eucharist; he does not have to 'renew' the Reserved Sacrament regularly because the regular use each Sunday of what was not consumed the previous Sunday does this automatically [except with regard to the Benediction Host]. But how sustainable theologically is the valde optandum in itself?
"So that, through signs, Communion may better appear a participation of the Sacrifice which is actually being celebrated". Thus I literally translate the IGMR explanation. I certainly have no hang-ups with this recommendation. I actually myself rather prefer to communicate the people without recourse to the Tabernacle. I have always found it a bit of a bother to have to shift the Altar Card and unlock the tabernacle with my thumb and forefinger conjoined (and I'm often in a strange church and the Tabernacle lock and key don't quite work as I expect them to). But I am unsure how much water the official explanation actually does hold. I would take as my starting point the principle that "the Sacrifice actually being celebrated" is not so much the Sacrifice of Fr X and the good people who have walked or driven to his church on that particular morning, as it is the One Sacrifice of Calvary; what those of us who are proud of our Anglican Patrimony instinctively think of as the Lord's One Oblation of Himself once offered, a full, perfect, and sufficient Sacrifice, Oblation, and Satisfaction: Christ's offering made sacramentally present upon our Altar. Each Mass is Calvary; there are neither two Calvaries nor many Calvaries.
I have a nagging suspicion that the modern valde optandum is part and parcel of the same sociological and anti-transcendent misunderstanding of the Eucharist fostered by and expressed in a desire for exclusively versus populum celebration. As Cardinal Ratzinger warned us, there is the danger of the inward-looking circle finding its own completion and fulness in its own activity rather than in the Lord who comes from beyond. There may in modern Liturgy be a subconscious fear of whatever does not come, self-generated, from within the Circle. There could even be the horrible superstition of laypeople being led to think that they ought to receive a host which they themselves had 'helped' to consecrate.
To be communicated from the Tabernacle with a host consecrated at a previous Mass reminds one that every Mass is the One Mass; that, as that great Separated Doctor of Catholic Truth (Fr Aidan Nichols' phrase), Eric 'Patrimony' Mascall, put it, a plurality of Masses is "the same thing - the same essentially, the same numerically - not just a lot of different things of the same kind, but the very same identical thing ... the one redemptive act which Christ, who died for our sins and rose again for our justification, perpetuates in the Church which is his Body through the Sacrament of his body and blood."
The ancient tradition of our Roman and Catholic Church does indeed encourage us to look beyond the closed circle of those gathered here and at this moment. When domnus papa sent the fermentum, the consecrated Host from his own Mass, to be mingled with the Masses of his presbyters in the outlying Roman churches as a sign of their unity with him, the separated congregations were thereby urged to look beyond their own closed circles. Furthermore, the Pope himself, as he approached the Altar at the start of his own Mass, was shown an opened capsa containing a consecrated particle from a previous Mass, which he adored. After the Pax he placed this particle in the chalice. (To this extent, Archimandrite Taft did not quite tell the whole truth when he claimed that " ... communion during Mass from hosts already consecrated at a previous Eucharist was totally unthinkable in the early Christian East and West".) Jungmann rightly explains: "In this way the continuous unity of the eucharistic sacrifice was expressed - the same Mass yesterday and today". The mingling, with our oblata, of hosts consecrated in other places or at other times, can be a powerful manifestation of the diachronic and synchronic identity of 'our' Mass with all the 'other' Masses which are being celebrated elsewhere and were celebrated at other times. For all those 'other' Masses, together with 'ours', are truly but the one Oblatio Domini.
We clergy should give appropriate respect to a recommendation of Ven Pius XII and of the IGMR when we are celebrating the Holy Mysteries. But a Catholic is not forbidden to argue that its rationale represents a dated and thoroughly unhelpful piece of thinking, which is likely to give layfolk the wrong end of more than one stick. After all, if liturgists in the pre-Conciliar period had never asked questions about the ruling praxis of their own day, there would have been no changes made in the 1960s! The heirs and admirers of those who then set aside the Liturgy of centuries are hardly well-positioned to assert that what was put in its place is beyond discussion!
Footnotes: (1) When Pius XII cited (CTS translation of Mediator Dei para 126) the Bull Certiores effecti of Benedict XIV (13 November 1742; Magnum Bullarium Romanum 1752 Luxemburg edition pars decima pp 117-8) he misrepresented that Pontiff. Benedict is there dealing with a controversy "de obligatione, qua sacerdotes Missas celebrantes adstringantur Eucharistiam ministrare intra easdem fidelibus iis, qui ad ipsam accipiendam paratos se exhibent, ac petunt sacrificii, cui adstant, participes fieri"; that is, he is dealing with the desire of the faithful to receive Communion within the Mass itself and not at some other time from the reserved sacrament. Benedict is not dealing with some convenient pastoral habit by which a tabernacle is kept charged with full ciboria for the convenient and frequent communion of large numbers. I doubt (think about it!) if that habit was common two centuries before S Pius X made frequent Communion the general practice.
(2) The CTS English translation of Mediator Dei by a Mgr G D Smith contains a curious omission of three words quoted by Pius XII from Benedict XIV: " ... quamvis de eodem sacrificio participent, praeter eos quibus a Sacerdote celebrante tribuitur in ipsa Missa portio victimae a se oblatae, ii etiam, quibus Sacerdos Eucharistiam reservari solitam ministrat ...". Perhaps (subconsciously?) realising that the words in ipsa Missa in fact subvert the sense which Pius XII is erroneously attributing to Benedict XIV, Smith left them out of his translation.
(3) The words "de eodem sacrificio participent" in the passage I quote from Benedict XIV in the previous footnote in fact support the point I make in the second and fourth paragraphs of my main article. Whether the Host you receive was consecrated at that Mass, or was reserved at a previous Mass, you still receive "from the same Sacrifice".
Revised with the help of readers from a much earlier draft.