I could not agree with Ross more. The greatest art is about the human condition, and I think the deeper this content, the finer the work of art. I don't think it matters, always, whether the creator is an illustrator or not. And it doesn't matter who's paying for the piece. What matters is the depth of expression.
Most "fine" artists, like Picasso, have produced some pretty poor pieces, and some illustrators like Rockwell have produced some very fine pieces.
Some might argue that a great deal of the "quality" is derived from how the paint has been applied. Rockwell did not do a great deal with his "surfaces." But art's quality should not be limited to surface texture.
Some works to illustrate my points. I've seen Hunt's Lady of Shallot and it's stunning.
William Holman Hunt Lady of Shallot and John William Waterhouse Lady of Shallot
These are affecting images. The second one I found in a search for the first one, but I also like it.
Even I, who loves abstraction, have to admit that this Picasso is less affecting than the other images above, although others may disagree.
I think a great deal of the "quality" has to do with the depth of the image. Sometimes illustrators deal with a single moment or image or gag, and it doesn't go much deeper than that. Rockwell, in his Four Freedoms, did go deeper, partly because of his profoundly patriotic inspiration. (Here's one of his studies for Freedom of Speech.) But other images seem more clever and humorous and less deep, as in his Sixth Inning below.
Whereas Michelangelo is always powerful because he is always dealing with the essential struggles of life, even in little details on the Sistine Chapel, as in Ezekial below.
