To see what was behind this edition, which prevailed for hundreds of years Trent, we turn to Johannes Berchmans Goeschl from the Summer issue of Sacred Music. He explains that the post-Trent reform efforts were driven by artistic issues rather than a concern for authenticity.
In the name of primacy of the text, the “reformers” wanted to purify the Gregorian chant of all offensive “barbarisms.” By “barbarism” they especially meant that in the inherited Gregorian chant, neume groups with several notes, or even melismas, are oftentimes placed on an unaccented syllable, while the accented syllables are oftentimes treated with only a single note. One aimed especially at the avoidance of melismas on short unaccented syllables immediately before the text accent, and also the simplification of concluding melismas.
Clarity and uniformity were to be achieved in the realm of modality, especially by ensuring that each chant begins with either the first scale degree or the reciting tone of the mode. In order to take into account modern sensibilities, many B-naturals were lowered to B-flat, and this often in modes for which the B-natural is essential to the character.
The author goes further to explains that "The melodic line of unison Gregorian chant in its most complete development in the Mass propers depends essentially upon ornamentation, even on unaccented syllables. This ornamentation need not be to the disadvantage of the text, but rather, it can bestow an all the more heightened profile upon the text. To have missed this was one of the most grave of the false interpretations of the authors of the Medici edition."
He provides Factus est Dominus, Introit for the Second Sunday after Pentecost, as an example, but there are examples in nearly every chant.
It is striking and, really, quite astounding. What is the lesson here? There are many of course but it does point to the dangers of focus exclusively on text at the expense of the reality that chant is, after all, music and art; text and music work together, and an exclusive emphasis on one at the expense of the other can lead to error.
In addition, there is a broader lesson that we must be careful in following the "style of our times" rather than the accumulated wisdom of the ages. Tradition is much "smarter" and more knowledgeable than any single generation. Rationalism tempts every generation, and later generations almost always curse the results.
A special thank you to Matthew Williams for his donation of the book for scanning!