Well said ubertuber, I couldn't agree with you more.
One point I could add as well - composition of new pieces isn't just "randomly thrown together," although I can understand why you might see it in this way. Since you (Dbass) mentioned John Cage in your initial post, I'll use him as an example. (and I like him anyway.
) In some of his indeterminatly composed pieces, he used the I Ching sticks to tell him how to write the piece. It tied in directly with his personal philosophy on music which was that it should be meditative in nature and calm the soul/sober the mind. So, while it might sound like a bunch of "junk" to you, it's really his personal philosophy combined with new experimental composition techniques at work together in a process which is actual quite involved and complicated.
I hope that makes at least a little sense, I should never be a music history teacher.