On April 3 1969, Pope Paul VI promulgated the Ordo Missa which was to be incorporated into the imminent edition of his Missale Romanum. In the accompanying Instructio Generalis, he included some remarks about when each of the four Eucharistic Prayers might or should be used.
With regard to EP I, the Roman Canon, he made a legislative statement which he did not make with regard to the other three Prayers. It was that it semper adhiberi potest. This has a very valuable consequence. It means that a priest who resolves that he will use that Prayer invariably cannot be accused of lacking the true mind of the Church, on the grounds that he never uses three other authorised Prayers. It means that when the same text goes on to suggest that Payer II is most suitable on weekdays, this cannot be held to render the use of the Canon Romanus on weekdays to be inappropriate. To use EP I invariably cannot be contrary to the spirit of the new Ordo Missae, because this provision explicitly sanctions such a resolve. Since this Ordo Missae states on its first page that it is ex decreto of the Second Ecumenical Vatican Council, it cannot, unless the Pope was lying, be contrary to the spirit of that Council invariably to use the Canon Romanus.
The Instructio Generalis also remarks that the Canon Romanus is used opportunius on the Sundays and Solemnities of the year; on the festivals of Saints whose names occur in that Prayer; and on days when Proper formulae are provided for the Communicantes or the Hanc igitur.
There are 52 Sundays in the year; and, by my rough estimate, 53 days covered by the other occasions thus listed. So, on between a quarter and a third of the days of the year ... and certainly on Sundays and Days of Obligation ... a strong preference for the use of the Canon Romanus is eminently in accordance with the expressed mind of the Legislator.
But what should become of the alternative Eucharistic Prayers? Are they fit for nothing but the dustbin? I am rather shy of such an attitude ... just as I feel that an earthenware 'chalice' which has been used for the Eucharist should not be rubbish. I am reminded of the suggestion made by Aidan Nichols in Looking at the Liturgy pp 121 sqq.; they are not really Roman; let them be a ritus communis with 'a multiple purpose in view' ... vide his suggestions.