Yeah, I know that sounds pretty inflammatory, especially coming from someone who kind of enjoys theory. I'm just going to quote a couple of passages from James McKinnon, musicologist and editor of The Early Christian Period and the Latin Middle Ages, and briefly comment.
Musica in Late Antiquity was not so much the everyday product of singing and playing that we call music today as it was the academic enterprise that we call music theory. Moreover, the music theory of the time was considerably more abstract than the effort that goes by that name in recent times; certainly nineteenth-century harmonic practice. In Late Antiquity the subject was permeated with Neoplatonic thinking, where ideas were considered to be real and where external manifestations of any sort--what we call reality--were mere shadows of those ideas. In this context the theoretical constructs themselves were the musical reality: good theory was the product of sophisticated mathematical calculation and the ingenioius manipulation of tonal symmetries.
And a couple pages later
...[Medieval music theorists] and their successors developed a body of music theory that not only described their musical practice in a consistent and systematic way, but set Western music on its peculiarly rational course.... [This rational course] permitted the eventual composition of great architectonic musical structures like those of the later eighteenth and earlier nineteenth centuries, but it may have forced Western music to sacrifice much of the rhythmic and tonal nuance that characterizes the musics of certain other high cultures.
It's fascinating to me that not much has changed in over a thousand years. First, the field of music theory is quite abstract today--so according to McKinnon, I guess I can only imagine how abstract it was in the first millenium C.E. Intellectual compartmentalization (McKinnon's term) is not only a hallmark of music theory, but of theoretical studies in any field within the Western intellectual context.
And that brings me to my second comment. Can we link the whole of Western thought from the Greeks to World War I and beyond to Modernism? Modernism is so inextricably linked with Western thought that the terms are nearly synonymous. Despite the past century's reactions against Modernism (eg. Postmodernism) and globalization, I suspect that Modernsism is so entrenched in Western minds that it will remain the default intellectual paradigm for decades (centuries?). (Here's a link to a chart contrasting Modernism with Postmodernism. Some problems, especially oversimplifications, but a goot place to start)
The good news is that Postmodernism with its outward-looking, inclusive nature is already influencing the abstraction of music theory. Nowadays one can easily find theoretical articles on music from around the world, including tuning systems, rhythms, and other musical elements not supported by Western theory and notation, as well as clear, practical applications of theorectical concepts that aid performers.