For years, I've heard that the rhythmic system of Dom Mocquereau has "no historical basis" (the quote is from the wildly misnamed book by Robert Fowells: Chant Made Simple (2000), a book on which I'll comment at greater length in some other post). This common statement raises a number of questions I won't go into here.
In any case, the indisputable facts are: no one has looked at as many original manuscripts as he did during his lifetime; his hand is the one behind the current chant editions we have to sing; his system of rhythm makes it possible for large groups of singers to sing together; his system makes sight reading the chant possible; his system makes it possible to liberate oneself from the dictate of an overweening directors who expect the singers to memorize their every idiosyncrasy; and his system is the only one that is thoroughly worked out, universalizable, and creates a beautiful result. For all these reasons, the Mocquereau system is the one used at most all seminaries in the world that teach the chant.
The opponents of Mocquereau have had a field day because his writings have been very hard to get. This is why I'm so pleased to announce that his master treatise, The Rhythm of Gregorian Chant, is now available from the CMAA in an easily downloadable PDF and also in a softcover, 440-page book. This is volume one. Volume two has never been translated and published in full.