Quel est selon vous le motif de la concélébration (c'est le mot, il n'y en pas d'autre) de la messe d'ordination?...

Je vous souffle la réponse (vous validerez doctement, je vous fais confiance) : manifester l'unité du sacerdoce du Christ entre l'évêque (seul détenteur de la plénitude de l'Ordre) et les nouveaux prêtres. Pareil comme pour la messe du Jeudi Saint (anniversaire de l'institution du sacerdoce sacramentel) dans le rite lyonnais, tiens... Et comme pour la messe chrismale. Quel est le problème, vraiment?

En passant, la concélébration existe chez les catholiques orientaux. Je vois mal comment un tradi peut en dire du mal (à condition d'être un peu cultivé, bien entendu). 1755, c'est quand même assez longtemps avant Vatican II...

"[It used to be] custom in both the western and the eastern church for the priests to offer Mass together with the bishop. The evidence is collected by Christianus Lupus in the Appendix on the Council of Chalcedon in his ad Concilia Generalia, et Provincialia, vol. 1, p. 994, of the first edition, where he interprets the words of Bassianus: "He used to celebrate Masses with me and to communicate with me"; and by Georgius, Lyturgiae Pontificiae, vol. 2, p. lf, and vol. 3, p. lf. The rite of concelebration is now out of fashion in the western church, except at priestly ordinations performed by the bishop and at episcopal consecrations where two bishops assist the consecrating bishop. But this rite continues to thrive in the Oriental Church, and priests often concelebrate with the bishop or with the priest as chief celebrant. This practice is derived from the Apostolic Constitutions, bk. 8, and the eighth Apostolic Canon. And wherever this custom is practiced among Greeks and Orientals, it is approved and to be preserved as We command in Our constitution Demandatam, sect. 9."

Benoit XIV, Allatae sunt (On the observance of Oriental Rites), n. 38.

Source: www.ewtn.com/library/encyc/b14allat.htm