In short, the appendix isn't TOO bad for us, so there's not a big enough reason for it to disappear. Other traits make a bigger difference. That source goes against what you said about it digesting cellulose ("But just because the appendix performs no digestive function"). And that was in my source wasn't it? lol... But anyway, the way I understand it is that there is lymphatic tissue all over the intestines (and I quote: "like much of the rest of the intestine"), and the appendix just happens to have some. Saying that the appendix itself is useful is like saying that since the ear has blood vessels it is therefore part of the circulatory system and it does serve a function. It just makes no sense.