I agree with pohnpei that ultimately it is the writing that counts. As a teacher, I would ideally like almost everyone to participate in class discussion. (I say "almost everyone," because occasionally those who talk the most have the least to contribute.) I realize, however, that some of the most thoughtful students are sometimes the most quiet. I try to do what I can to get them to talk, but, if they don't want to talk, I don't force them. I had such a student this past term; he never said a word, but his writing was superb. He received a strong "A."
Once I had such a student. He was obviously intelligent, had obviously read, and wrote beautifully, yet he never spoke. To tease him one day, when we were discussing Paradise Lost I suddenly asked him, "what would you YOU do if YOU were in hell?" Without missing a beat, he promptly replied, "Stop, drop, and roll." I have never forgotten that moment!