You have to read to understand them, don't you? On the other hand, you would probably get an F if you used one for a book report in English class. Also, in one sense literature can be any novel, but if you go to Barnes & Noble and look in the literature section you will find what is called classic literature. It all depends on how you decide to define it. It's a matter of semantics.
BTW, don't let anyone tell you this is a lesser form of reading. Novels have their own appeal. Comics and manga are admittedly light reading, but they have their own merits, merits not shared quite the same way with any other medium. Besides, a reader is a reader. I don't know anyone who sticks to just comics for their fiction, it's only natural to expand your interests, and if not, so what? Comics, which includes manga (though I really wish Yahoo would put them in separate categories) can target any age or level of intelligence desired, there is nothing inherent in the medium itself that would somehow make it less legitimate or deserving of recognition. It's in the hands of the creators.