That, however, is rubbish.
Great works of literature are great because they merit such distinction--not because they happen to be of one particular genre and not another. We don't look down our nose at War and Peace because it's a novel and not poetry. We don't think less of King's "Letter from Birmingham Jail" because it's "only a letter."
Visual narrative is a genre worthy of study, both for ourselves and for our students. Comics in Education (www.comicsineducation.com) is dedicated to proving that.