The other thing that makes theory of art redundant for the artists is that frankly, most theories boil down to more or less the same things. Every one wants to create their own artistic language; everyone tries to create something predicated upon the idea of the beautiful, whether positively or negatively; everyone needs also some organizing concept or methodology to hold their ideas together that depends on some kind of logic.
For me, differences in art boils down to the type of humans they speak to. You can't have Virgil nowadays, the same way you won't have Picasso during the Stone Age. And I don't mean it in the individual sense: good art is what I like; bad art is what you like. I mean it in the collective sense: the complex of social practices, the experience of life (you can't have Guernica outside of the Guernica massacre; no James Joyce without the modern capitalistic age). And I don't mean either to say that lyric expression is bad: lyric expression speaks to the individual who lives in a real world. But in the end a work of art always link back to a more general experience of life.
I agree heartily with John, because even though I managed to find some great 20thC "serious" music (and that include John Cage, Arvo Pärt and Sonic Youth), I despise profundly the systematizers because any system of art is arbitrary, and thus creating a new one means nothing in itself.