A few points that may focus the discussion here:
-From John Rahn's "Basic Atonal Theory":
Thirdly, not all atonal music is good music. In fact, most contemporary music is not very good. The average concert of newly written music will include several pieces which are mediocre, if not just plain awful. This has been the case for newly written music of all historical periods, and the present is no exception.
Often, a performance of a contemporary piece of music receives one of two reactions:
1) I don't like this; modern music is awful/ is irrelevant/ is pretentious
2) I don't understand this, so I'm inferior and need to study up
The first is egotistical, and the second uses inverted logic. I contend that there is a third possibility, which is simply that the particular piece that you heard was a bad piece. Statistically this is the most likely possibility.
-While this thread is purportedly about "modern classical music", it references only to a very specific period in the 20th century before 1950. Please keep in mind that many composers of this time period, primarily as an attempt to insulate the western tradition from jazz influences (among other reasons), purposefully made their pieces to be displeasing in a conventional sense. This one span of music history does not represent the entire 20th and 21st centuries.
-Probably none of us has heard, digested, analyzed, and performed enough contemporary music to pass such sweeping judgments on it, as if it were a single phenomenon in the first place. Such judgments are infinitely more arrogant than writing any music, "theoretical" or otherwise, ever could be.
-Our personal or the "audience's" enjoyment of a piece is no measure of its value.
-Calling something, awful, pretentious, disorganized, or theoretical does not count as an argument, or even evidence.