If the purpose of literature is to convey a (often philosophical) message, it generally doesn't do a very good job of it. Either the message becomes obscured (deliberately, it seems) by the fictional story it's buried in, or the author belabors the message to the point of annoyance, or both. If you as an author want to convey a message, why shroud it in a long-winded story? Just come right out with it. Write a philosophical work and spare your readers the trouble of slogging through rambling dialogue and unnecessary detail.
As a reader, if you want philosophy, read philosophy. If you want entertainment, read books meant to entertain. When you read books that try to do both, you'll probably end up bored and annoyed like me.
I picked up The Brothers Karamazov by Fyodor Dostoevsky a couple weeks ago and I'm about half way through the novel. The story is not entertaining, there's tons of fluff, and the philosophical message is belabored. Yes, Dostoevsky, we understand that you believe that simple faith trumps analytical skepticism. We see over and over how the novel's characters of faith (Father Zossima, Alyosha) have a positive effect on the world around them while the characters of doubt (Fyodor Pavlovitch, Dmitri, Ivan) end up destroying themselves and those around them. That's all well and good, but what have you got to back up your philosophy? Who says non-believers can't have a positive effect on the world? This is, after all, a work of fiction, where the author can manipulate the story in whichever way he pleases to support his particular philosophy.
As you can see, I'm a little frustrated with literature as a whole right now. Is there something I'm missing, some perspective on literature I haven't considered that could possibly redeem it?