The following is from "The Obligation of the Clergy to Recite the Divine Office" by Thomas J. Williams, originally published in American Church Quarterly in 1930. It can be found here at Project Canterbury.
We are now faced with the contention of those who admit that the Prayer-Book Offices are of obligation for Priests and Deacons of the Church in England, by force of explicit enactment; but who claim that the failure of the American Church, in 1790, to repeat the requirement of the English Prayer-Book in explicit terms, abrogates for the clergy of the American Church the specific obligation of reciting Daily Morning and Evening Prayer, leaving us free to choose the form or rite we shall use in fulfilling our obligation as Catholic Priests to say the Divine Office. This contention is based on the argument from silence--an argument that can cut like a two edged sword, and has been known to cut both ways. It is freely granted that the revisers of 1790 did not explicitly reenact or refer directly to the requirement of the English Prayer-Book that the clergy shall recite the Divine Office each day. But the designation of the offices in the American Prayer-Book, since its first ratification in 1790, as "The Order of Daily Morning Prayer" and "The Order of Daily Evening Prayer," is to be interpreted in the light of the statement of the Preface to the American Prayer-Book, that "this Church is far from intending to depart from the Church of England in any essential point of doctrine, discipline, or worship." The requirement of daily recitation by the clergy of the Divine Office is certainly an essential point of discipline and worship, inasmuch as all clerks in Holy Orders, of whatever Communion of the Holy Catholic Church, are obligated to such recitation. No one will deny that the clergy of the Roman Communion are under strict obligation to use the offices of the Roman, or other authorized, Breviary--and none other. It should be equally clear that all Priests and Deacons of the American Church are under obligation to say the Divine Office, as set forth in the Order of Daily Morning and Evening Prayer; and have no right to substitute for these authorized offices the Roman Breviary or the Orthodox Horologion.
It has been the practice of an almost unbroken line of Anglican clergy, from the Reformation to the present, to supplement the Prayer-Book Office by reciting the little hours of the old office. Such practice does not admit of question or challenge, for this has always been a matter of private devotion. Entirely different is the practice of substituting the entire Breviary for the Prayer-Book Office. Whatever an individual priest, or a community of priests, may find helpful as a matter of individual or community devotion, this can in no wise affect the obligation resting on every Priest and Deacon of the Anglican Rite, as such, to recite the Divine Office according to the authorized form set forth by authority--The Order of Daily Morning and Evening Prayer.