Mark Shea posts a rant on modern church art. He is a little rough on the artist, but I can't fault him too much for that... something really must be done, and sometimes that means shaking people out of their complacency. Art is important, and when we fail at art it is a sign we have failed at some very deep, foundational level.
Steven Graydanus weighs in in the combox and asks the centrally important question of what is gained when religious art jettisons its representational aspect.
See, that's the rub. Modern art focuses on reductionism, deconstruction and isolation, teasing out the various *elements* of art and treating them as if they have been the whole point of art all along... as if the cave painters of Lascaux were all a-twitter over "composition" or "color schemes".
As SDG points out above, this is all a completely negative process. Any decent traditional stained glass window will have all the beautiful colors with the light streaming through *AND* will also have the narrative element that modern abstraction lacks. It's like a two-fer!
It is also part and parcel of the mystery of art. The idea that we can represent some thing or story by means of an arrangement of lines and color and shade is powerful magic, like the mystery of verbal language. You pick up a book, scan some marks on the page, and worlds are opened. A painting is never about mere "marks" or "color", any more than a book is about "paragraphs" or "pages". Nobody reads a book because they love "chapters".
Not that there is no place for abstraction. Chartres cathedral is chock full of abstraction, interwoven with the figurative images, but it is an orderly and balanced abstraction, not the randomized stuff we see so commonly now. Order is a good thing in itself, and there is in the Rose Window a certain playfulness within the order that could never be discerned in a the random scatterings of color we now see put forward as "serious" art.
So, what does a stained glass window gain by losing the figures, the narrative and the order? Nothing. It gains nothing, and loses nearly everything. But working with narrative and doing a good job of it (not kitsch) is hard, and the modern abstractions are far easier. That's what it comes down to.
Incidentally, there has been a lot of theorizing about just why the cave painters at Lascaux and elsewhere painted their pictures, and these usually have a lot to do with completely specualtive and unproveable ideas about conjuring animal spirits or invoking the gods of the hunt, or some such. I will throw in, however, with G.K. Chesterton, who boldly theorized that the cave painters painted for the same reason that I do... because it was fun.